ParseStmt.cpp

//===--- ParseStmt.cpp - Swift Language Parser for Statements -------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// Statement Parsing and AST Building
//
//===----------------------------------------------------------------------===//

#include "swift/AST/ASTWalker.h"
#include "swift/AST/Attr.h"
#include "swift/AST/AvailabilitySpec.h"
#include "swift/AST/Decl.h"
#include "swift/AST/FileUnit.h"
#include "swift/Basic/Assertions.h"
#include "swift/Basic/Defer.h"
#include "swift/Basic/Version.h"
#include "swift/Parse/IDEInspectionCallbacks.h"
#include "swift/Parse/Lexer.h"
#include "swift/Parse/Parser.h"
#include "swift/Subsystems.h"
#include "llvm/ADT/PointerUnion.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/SaveAndRestore.h"

using namespace swift;

/// isStartOfStmt - Return true if the current token starts a statement.
///
bool Parser::isStartOfStmt(bool preferExpr) {
  // This needs to be kept in sync with `Parser::parseStmt()`. If a new token
  // kind is accepted here as start of statement, it should also be handled in
  // `Parser::parseStmt()`.
  switch (Tok.getKind()) {
  default: return false;
  case tok::kw_return:
  case tok::kw_throw:
  case tok::kw_defer:
  case tok::kw_if:
  case tok::kw_guard:
  case tok::kw_while:
  case tok::kw_do:
  case tok::kw_for:
  case tok::kw_break:
  case tok::kw_continue:
  case tok::kw_fallthrough:
  case tok::kw_switch:
  case tok::kw_case:
  case tok::kw_default:
  case tok::kw_yield:
  case tok::kw_discard:
  case tok::pound_assert:
  case tok::pound_if:
  case tok::pound_warning:
  case tok::pound_error:
  case tok::pound_sourceLocation:
    return true;

  case tok::kw_repeat:
    // 'repeat' followed by anything other than a brace stmt
    // is a pack expansion expression.
    // FIXME: 'repeat' followed by '{' could be a pack expansion
    // with a closure pattern.
    return peekToken().is(tok::l_brace);

  case tok::pound_line:
    // #line at the start of a line is a directive, when within, it is an expr.
    return Tok.isAtStartOfLine();

  case tok::kw_try: {
    // "try" cannot actually start any statements, but we parse it there for
    // better recovery in cases like 'try return'.

    // For 'if', 'switch', and 'do' we can parse as an expression.
    if (peekToken().isAny(tok::kw_if, tok::kw_switch) ||
        (peekToken().is(tok::kw_do) &&
         Context.LangOpts.hasFeature(Feature::DoExpressions))) {
      return false;
    }
    Parser::BacktrackingScope backtrack(*this);
    consumeToken(tok::kw_try);
    return isStartOfStmt(preferExpr);
  }
      
  case tok::identifier: {
    // "identifier ':' for/while/do/switch" is a label on a loop/switch.
    if (!peekToken().is(tok::colon)) {
      // "yield", "discard", and "then" in the right context begins a statement.
      if (isContextualYieldKeyword() || isContextualDiscardKeyword() ||
          isContextualThenKeyword(preferExpr)) {
        return true;
      }
      return false;
    }

    // To disambiguate other cases of "identifier :", which might be part of a
    // question colon expression or something else, we look ahead to the second
    // token.
    Parser::BacktrackingScope backtrack(*this);
    consumeToken(tok::identifier);
    consumeToken(tok::colon);

    // We treating IDENTIFIER: { as start of statement to provide missed 'do'
    // diagnostics. This case will be handled in parseStmt().
    if (Tok.is(tok::l_brace)) {
      return true;
    }
    // For better recovery, we just accept a label on any statement.  We reject
    // putting a label on something inappropriate in parseStmt().
    return isStartOfStmt(preferExpr);
  }

  case tok::at_sign: {
    // Might be a statement or case attribute. The only one of these we have
    // right now is `@unknown default`, so hardcode a check for an attribute
    // without any parens.
    if (!peekToken().is(tok::identifier))
      return false;
    Parser::BacktrackingScope backtrack(*this);
    consumeToken(tok::at_sign);
    consumeToken(tok::identifier);
    return isStartOfStmt(preferExpr);
  }
  }
}

ParserStatus Parser::parseExprOrStmt(ASTNode &Result) {
  if (Tok.is(tok::semi)) {
    diagnose(Tok, diag::illegal_semi_stmt)
      .fixItRemove(SourceRange(Tok.getLoc()));
    consumeToken();
    return makeParserError();
  }

  if (Tok.is(tok::pound) && Tok.isAtStartOfLine() &&
      peekToken().is(tok::code_complete)) {
    consumeToken();
    if (CodeCompletionCallbacks) {
      CodeCompletionCallbacks->completeAfterPoundDirective();
    }
    consumeToken(tok::code_complete);
    return makeParserCodeCompletionStatus();
  }

  if (isStartOfStmt(/*preferExpr*/ false)) {
    ParserResult<Stmt> Res = parseStmt();
    if (Res.isNonNull()) {
      auto *S = Res.get();

      // Special case: An 'if' or 'switch' statement followed by an 'as' must
      // be an if/switch expression in a coercion.
      // We could also achieve this by more eagerly attempting to parse an 'if'
      // or 'switch' as an expression when in statement position, but that
      // could result in less useful recovery behavior.
      if ((isa<IfStmt>(S) || isa<SwitchStmt>(S) ||
           ((isa<DoStmt>(S) || isa<DoCatchStmt>(S)) &&
            Context.LangOpts.hasFeature(Feature::DoExpressions))) &&
          Tok.is(tok::kw_as)) {
        auto *SVE = SingleValueStmtExpr::createWithWrappedBranches(
            Context, S, CurDeclContext, /*mustBeExpr*/ true);
        auto As = parseExprAs();
        if (As.isParseErrorOrHasCompletion())
          return As;

        Result = SequenceExpr::create(Context, {SVE, As.get(), As.get()});
      } else {
        Result = S;
      }
    }
    return Res;
  }

  // Note that we're parsing a statement.
  StructureMarkerRAII ParsingStmt(*this, Tok.getLoc(),
                                  StructureMarkerKind::Statement);

  if (CodeCompletionCallbacks) {
    CodeCompletionCallbacks->setExprBeginning(getParserPosition());
  }

  if (Tok.is(tok::code_complete)) {
    auto *CCE = new (Context) CodeCompletionExpr(Tok.getLoc());
    Result = CCE;
    if (CodeCompletionCallbacks) {
      CodeCompletionCallbacks->completeStmtOrExpr(CCE);
    }
    consumeToken(tok::code_complete);
    return makeParserCodeCompletionStatus();
  }

  ParserResult<Expr> ResultExpr = parseExpr(diag::expected_expr);
  if (ResultExpr.isNonNull()) {
    Result = ResultExpr.get();
  } else if (!ResultExpr.hasCodeCompletion()) {
    // If we've consumed any tokens at all, build an error expression
    // covering the consumed range.
    SourceLoc startLoc = StructureMarkers.back().Loc;
    if (startLoc != Tok.getLoc()) {
      Result = new (Context) ErrorExpr(SourceRange(startLoc, PreviousLoc));
    }
  }

  return ResultExpr;
}

/// Returns whether the parser's current position is the start of a switch case,
/// given that we're in the middle of a switch already.
static bool isAtStartOfSwitchCase(Parser &parser,
                                  bool needsToBacktrack = true) {
  std::optional<Parser::BacktrackingScope> backtrack;

  // Check for and consume attributes. The only valid attribute is `@unknown`
  // but that's a semantic restriction.
  while (parser.Tok.is(tok::at_sign)) {
    if (!parser.peekToken().is(tok::identifier))
      return false;

    if (needsToBacktrack && !backtrack)
      backtrack.emplace(parser);

    parser.consumeToken(tok::at_sign);
    parser.consumeToken(tok::identifier);
    if (parser.Tok.is(tok::l_paren))
      parser.skipSingle();
  }

  return parser.Tok.isAny(tok::kw_case, tok::kw_default);
}

bool Parser::isTerminatorForBraceItemListKind(BraceItemListKind Kind,
                                              ArrayRef<ASTNode> ParsedDecls) {
  switch (Kind) {
  case BraceItemListKind::Brace:
  case BraceItemListKind::TopLevelCode:
  case BraceItemListKind::TopLevelLibrary:
  case BraceItemListKind::MacroExpansion:
    return false;
  case BraceItemListKind::Case: {
    if (Tok.is(tok::pound_if)) {
      // Backtracking scopes are expensive, so avoid setting one up if possible.
      Parser::BacktrackingScope Backtrack(*this);
      // '#if' here could be to guard 'case:' or statements in cases.
      // If the next non-directive line starts with 'case' or 'default', it is
      // for 'case's.
      do {
        consumeToken();

        // just find the end of the line
        skipUntilTokenOrEndOfLine(tok::NUM_TOKENS);
      } while (Tok.isAny(tok::pound_if, tok::pound_elseif, tok::pound_else));
      return isAtStartOfSwitchCase(*this, /*needsToBacktrack*/false);
    }
    return isAtStartOfSwitchCase(*this);
  }
  case BraceItemListKind::ActiveConditionalBlock:
  case BraceItemListKind::InactiveConditionalBlock:
    return Tok.isNot(tok::pound_else) && Tok.isNot(tok::pound_endif) &&
           Tok.isNot(tok::pound_elseif);
  }

  llvm_unreachable("Unhandled BraceItemListKind in switch.");
}

void Parser::consumeTopLevelDecl(ParserPosition BeginParserPosition,
                                 TopLevelCodeDecl *TLCD) {
  SourceLoc EndLoc = PreviousLoc;
  backtrackToPosition(BeginParserPosition);
  SourceLoc BeginLoc = Tok.getLoc();
  State->setIDEInspectionDelayedDeclState(
      SourceMgr, L->getBufferID(),
      IDEInspectionDelayedDeclKind::TopLevelCodeDecl, TLCD, {BeginLoc, EndLoc},
      BeginParserPosition.PreviousLoc);

  // Skip the rest of the file to prevent the parser from constructing the AST
  // for it.  Forward references are not allowed at the top level.
  while (!Tok.is(tok::eof))
    consumeToken();
}

///   brace-item:
///     decl
///     expr
///     stmt
///   stmt:
///     ';'
///     stmt-assign
///     stmt-if
///     stmt-guard
///     stmt-for-c-style
///     stmt-for-each
///     stmt-switch
///     stmt-control-transfer
///  stmt-control-transfer:
///     stmt-return
///     stmt-break
///     stmt-continue
///     stmt-fallthrough
///   stmt-assign:
///     expr '=' expr
ParserStatus Parser::parseBraceItems(SmallVectorImpl<ASTNode> &Entries,
                                     BraceItemListKind Kind,
                                     BraceItemListKind ConditionalBlockKind,
                                     bool &IsFollowingGuard) {
  bool IsTopLevel = (Kind == BraceItemListKind::TopLevelCode) ||
                    (Kind == BraceItemListKind::TopLevelLibrary ||
                    (Kind == BraceItemListKind::MacroExpansion &&
                     isa<FileUnit>(CurDeclContext)));
  bool isActiveConditionalBlock =
    ConditionalBlockKind == BraceItemListKind::ActiveConditionalBlock;
  bool isConditionalBlock = isActiveConditionalBlock ||
    ConditionalBlockKind == BraceItemListKind::InactiveConditionalBlock;

  ParserStatus BraceItemsStatus;

  bool PreviousHadSemi = true;
  while ((IsTopLevel || Tok.isNot(tok::r_brace)) &&
         Tok.isNot(tok::pound_endif) &&
         Tok.isNot(tok::pound_elseif) &&
         Tok.isNot(tok::pound_else) &&
         Tok.isNot(tok::eof) &&
         !isStartOfSILDecl() &&
         (isConditionalBlock ||
          !isTerminatorForBraceItemListKind(Kind, Entries))) {

    if (Tok.is(tok::r_brace)) {
      assert(IsTopLevel);
      diagnose(Tok, diag::extra_rbrace)
        .fixItRemove(Tok.getLoc());
      consumeToken();
      continue;
    }

    // Eat invalid tokens instead of allowing them to produce downstream errors.
    if (Tok.is(tok::unknown)) {
      if (startsWithMultilineStringDelimiter(Tok)) {
        // This was due to unterminated multi-line string.
        IsInputIncomplete = true;
      }
      consumeToken();
      continue;
    }
           
    bool NeedParseErrorRecovery = false;
    ASTNode Result;

    // If the previous statement didn't have a semicolon and this new
    // statement doesn't start a line, complain.
    const bool IsAtStartOfLineOrPreviousHadSemi =
        PreviousHadSemi || Tok.isAtStartOfLine();
    if (!IsAtStartOfLineOrPreviousHadSemi) {
      SourceLoc EndOfPreviousLoc = getEndOfPreviousLoc();
      diagnose(EndOfPreviousLoc, diag::statement_same_line_without_semi)
        .fixItInsert(EndOfPreviousLoc, ";");
      // FIXME: Add semicolon to the AST?
    }

    ParserPosition BeginParserPosition;
    if (isIDEInspectionFirstPass())
      BeginParserPosition = getParserPosition();

    // Parse the decl, stmt, or expression.
    PreviousHadSemi = false;
    if (Tok.is(tok::pound_if) && !isStartOfSwiftDecl()) {
      SmallVector<ASTNode, 16> activeElements;
      auto IfConfigResult = parseIfConfig(
          IfConfigContext::BraceItems,
          [&](bool IsActive) {
            SmallVector<ASTNode, 16> elements;
            parseBraceItems(elements, Kind,
                            IsActive
                                ? BraceItemListKind::ActiveConditionalBlock
                                : BraceItemListKind::InactiveConditionalBlock,
                            IsFollowingGuard);

            if (IsActive)
              activeElements.append(elements);
          });
      if (IfConfigResult.hasCodeCompletion() && isIDEInspectionFirstPass()) {
        consumeDecl(BeginParserPosition, IsTopLevel);
        return IfConfigResult;
      }
      BraceItemsStatus |= IfConfigResult;
      if (IfConfigResult.isError()) {
        NeedParseErrorRecovery = true;
        continue;
      }

      Entries.append(activeElements);
    } else if (Tok.is(tok::pound_line)) {
      ParserStatus Status = parseLineDirective(true);
      BraceItemsStatus |= Status;
      NeedParseErrorRecovery = Status.isErrorOrHasCompletion();
    } else if (Tok.is(tok::pound_sourceLocation)) {
      ParserStatus Status = parseLineDirective(false);
      BraceItemsStatus |= Status;
      NeedParseErrorRecovery = Status.isErrorOrHasCompletion();
    } else if (isStartOfSwiftDecl()) {
      SmallVector<Decl*, 8> TmpDecls;
      ParserStatus DeclResult =
          parseDecl(IsAtStartOfLineOrPreviousHadSemi,
                    /*IfConfigsAreDeclAttrs=*/true, [&](Decl *D) {
                      TmpDecls.push_back(D);

                      // Any function after a 'guard' statement is marked as
                      // possibly having local captures. This allows SILGen
                      // to correctly determine its capture list, since
                      // otherwise it would be skipped because it is not
                      // defined inside a local context.
                      if (IsFollowingGuard)
                        if (auto *FD = dyn_cast<FuncDecl>(D))
                          FD->setHasTopLevelLocalContextCaptures();
                    });
      BraceItemsStatus |= DeclResult;
      if (DeclResult.isErrorOrHasCompletion()) {
        NeedParseErrorRecovery = true;
        if (DeclResult.hasCodeCompletion() && IsTopLevel &&
            isIDEInspectionFirstPass()) {
          consumeDecl(BeginParserPosition, IsTopLevel);
          return DeclResult;
        }
      }
      Entries.append(TmpDecls.begin(), TmpDecls.end());

      // HACK: If any declarations were parsed, make the last one the "result".
      // We should know whether this was in an #if or not.
      if (!TmpDecls.empty()) {
        Result = TmpDecls.back();
      }

    } else if (IsTopLevel) {
      // If this is a statement or expression at the top level of the module,
      // Parse it as a child of a TopLevelCodeDecl.
      auto *TLCD = new (Context) TopLevelCodeDecl(CurDeclContext);
      ContextChange CC(*this, TLCD);
      SourceLoc StartLoc = Tok.getLoc();

      // Expressions can't begin with a closure literal at statement position.
      // This prevents potential ambiguities with trailing closure syntax.
      if (Tok.is(tok::l_brace)) {
        diagnose(Tok, diag::statement_begins_with_closure);
      }

      ParserStatus Status = parseExprOrStmt(Result);
      BraceItemsStatus |= Status;
      if (Status.hasCodeCompletion() && isIDEInspectionFirstPass()) {
        consumeTopLevelDecl(BeginParserPosition, TLCD);
        auto Brace = BraceStmt::create(Context, StartLoc, {}, PreviousLoc);
        TLCD->setBody(Brace);
        Entries.push_back(TLCD);
        return Status;
      }
      if (Status.isErrorOrHasCompletion())
        NeedParseErrorRecovery = true;
      else if (!allowTopLevelCode()) {
        diagnose(StartLoc,
                 Result.is<Stmt*>() ? diag::illegal_top_level_stmt
                                    : diag::illegal_top_level_expr);
      }

      if (!Result.isNull()) {
        // NOTE: this is a 'virtual' brace statement which does not have
        //       explicit '{' or '}', so the start and end locations should be
        //       the same as those of the result node, plus any junk consumed
        //       afterwards
        auto Brace = BraceStmt::create(Context, Result.getStartLoc(),
                                       Result, PreviousLoc, /*Implicit=*/true);
        TLCD->setBody(Brace);
        Entries.push_back(TLCD);

        // A top-level 'guard' statement can introduce local bindings, so we
        // must mark all functions following one. This makes them behave
        // as if they were in local context for the purposes of capture
        // emission in SILGen.
        if (auto *stmt = Result.dyn_cast<Stmt *>())
          if (isa<GuardStmt>(stmt))
            IsFollowingGuard = true;
      }
    } else {
      ParserStatus ExprOrStmtStatus = parseExprOrStmt(Result);
      BraceItemsStatus |= ExprOrStmtStatus;
      if (ExprOrStmtStatus.isError())
        NeedParseErrorRecovery = true;
      if (!Result.isNull())
        Entries.push_back(Result);
    }

    if (!NeedParseErrorRecovery && Tok.is(tok::semi)) {
      PreviousHadSemi = true;
      if (auto *E = Result.dyn_cast<Expr*>())
        E->TrailingSemiLoc = consumeToken(tok::semi);
      else if (auto *S = Result.dyn_cast<Stmt*>())
        S->TrailingSemiLoc = consumeToken(tok::semi);
      else if (auto *D = Result.dyn_cast<Decl*>())
        D->TrailingSemiLoc = consumeToken(tok::semi);
      else
        assert(!Result && "Unsupported AST node");
    }

    if (NeedParseErrorRecovery) {
      // If we had a parse error, skip to the start of the next stmt or decl.
      //
      // It would be ideal to stop at the start of the next expression (e.g.
      // "X = 4"), but distinguishing the start of an expression from the middle
      // of one is "hard".
      skipUntilDeclStmtRBrace();

      // If we have to recover, pretend that we had a semicolon; it's less
      // noisy that way.
      PreviousHadSemi = true;
    }
  }

  return BraceItemsStatus;
}

/// Recover from a 'case' or 'default' outside of a 'switch' by consuming up to
/// the next ':' or '}'.
static ParserResult<Stmt> recoverFromInvalidCase(Parser &P) {
  assert(P.Tok.is(tok::kw_case) || P.Tok.is(tok::kw_default)
         && "not case or default?!");
  P.diagnose(P.Tok, diag::case_outside_of_switch, P.Tok.getText());
  P.skipUntil(tok::colon, tok::r_brace);
  // FIXME: Return an ErrorStmt?
  return nullptr;
}

/// parseStmt
ParserResult<Stmt> Parser::parseStmt() {
  AssertParserMadeProgressBeforeLeavingScopeRAII apmp(*this);

  // If this is a label on a loop/switch statement, consume it and pass it into
  // parsing logic below.
  LabeledStmtInfo LabelInfo;
  if (Tok.is(tok::identifier) && peekToken().is(tok::colon)) {
    LabelInfo.Loc = consumeIdentifier(LabelInfo.Name,
                                      /*diagnoseDollarPrefix=*/true);
    consumeToken(tok::colon);
  }

  // Note that we're parsing a statement.
  StructureMarkerRAII ParsingStmt(*this, Tok.getLoc(),
                                  StructureMarkerKind::Statement);

  SourceLoc tryLoc;
  (void)consumeIf(tok::kw_try, tryLoc);

  // Claim contextual statement keywords now that we've committed
  // to parsing a statement.
  if (isContextualYieldKeyword()) {
    Tok.setKind(tok::kw_yield);
  } else if (isContextualDiscardKeyword()) {
    Tok.setKind(tok::kw_discard);
  }

  if (isContextualThenKeyword(/*preferExpr*/ false))
    Tok.setKind(tok::kw_then);

  // This needs to handle everything that `Parser::isStartOfStmt()` accepts as
  // start of statement.
  switch (Tok.getKind()) {
  case tok::pound_line:
  case tok::pound_sourceLocation:
  case tok::pound_if:
  case tok::pound_error:
  case tok::pound_warning:
    assert((LabelInfo || tryLoc.isValid()) &&
           "unlabeled directives should be handled earlier");
    // Bailout, and let parseBraceItems() parse them.
    LLVM_FALLTHROUGH;
  default:
    diagnose(Tok, tryLoc.isValid() ? diag::expected_expr : diag::expected_stmt);
    if (Tok.is(tok::at_sign)) {
      // Recover from erroneously placed attribute.
      consumeToken(tok::at_sign);
      consumeIf(tok::identifier);
    }
    return nullptr;
  case tok::kw_return:
    if (LabelInfo) diagnose(LabelInfo.Loc, diag::invalid_label_on_stmt);
    return parseStmtReturn(tryLoc);
  case tok::kw_yield:
    if (LabelInfo) diagnose(LabelInfo.Loc, diag::invalid_label_on_stmt);
    return parseStmtYield(tryLoc);
  case tok::kw_then:
    if (LabelInfo) diagnose(LabelInfo.Loc, diag::invalid_label_on_stmt);
    return parseStmtThen(tryLoc);
  case tok::kw_throw:
    if (LabelInfo) diagnose(LabelInfo.Loc, diag::invalid_label_on_stmt);
    return parseStmtThrow(tryLoc);
  case tok::kw_defer:
    if (LabelInfo) diagnose(LabelInfo.Loc, diag::invalid_label_on_stmt);
    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
    return parseStmtDefer();
  case tok::kw_if:
    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
    return parseStmtIf(LabelInfo);
  case tok::kw_guard:
    if (LabelInfo) diagnose(LabelInfo.Loc, diag::invalid_label_on_stmt);
    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
    return parseStmtGuard();
  case tok::kw_while:
    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
    return parseStmtWhile(LabelInfo);
  case tok::kw_repeat:
    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
    return parseStmtRepeat(LabelInfo);
  case tok::kw_do:
    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
    return parseStmtDo(LabelInfo);
  case tok::kw_for:
    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
    return parseStmtForEach(LabelInfo);
  case tok::kw_discard:
    if (LabelInfo) diagnose(LabelInfo.Loc, diag::invalid_label_on_stmt);
    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
    return parseStmtDiscard();
  case tok::kw_switch:
    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
    return parseStmtSwitch(LabelInfo);
  /// 'case' and 'default' are only valid at the top level of a switch.
  case tok::kw_case:
  case tok::kw_default:
    return recoverFromInvalidCase(*this);
  case tok::kw_break:
    if (LabelInfo) diagnose(LabelInfo.Loc, diag::invalid_label_on_stmt);
    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
    return parseStmtBreak();
  case tok::kw_continue:
    if (LabelInfo) diagnose(LabelInfo.Loc, diag::invalid_label_on_stmt);
    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
    return parseStmtContinue();
  case tok::kw_fallthrough: {
    if (LabelInfo) diagnose(LabelInfo.Loc, diag::invalid_label_on_stmt);
    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());

    auto loc = consumeToken(tok::kw_fallthrough);
    return makeParserResult(FallthroughStmt::createParsed(loc, CurDeclContext));
  }
  case tok::pound_assert:
    if (LabelInfo) diagnose(LabelInfo.Loc, diag::invalid_label_on_stmt);
    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
    return parseStmtPoundAssert();
  case tok::l_brace:
    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
    SourceLoc colonLoc = Tok.getLoc();
    diagnose(colonLoc, diag::labeled_block_needs_do)
      .fixItInsert(colonLoc, "do ");
    return parseStmtDo(LabelInfo, /*shouldSkipDoTokenConsume*/ true);
  }
}

/// parseBraceItemList - A brace enclosed expression/statement/decl list.  For
/// example { 1; 4+5; } or { 1; 2 }.  Always occurs as part of some other stmt
/// or decl.
///
///   brace-item-list:
///     '{' brace-item* '}'
///
ParserResult<BraceStmt> Parser::parseBraceItemList(Diag<> ID) {
  if (Tok.isNot(tok::l_brace)) {
    diagnose(Tok, ID);

    // Attempt to recover by looking for a left brace on the same line
    if (!skipUntilTokenOrEndOfLine(tok::l_brace, tok::r_brace) ||
        !Tok.is(tok::l_brace))
      return nullptr;
  }
  SourceLoc LBLoc = consumeToken(tok::l_brace);

  SmallVector<ASTNode, 16> Entries;
  SourceLoc RBLoc;

  ParserStatus Status = parseBraceItems(Entries, BraceItemListKind::Brace,
                                        BraceItemListKind::Brace);
  if (!parseMatchingToken(tok::r_brace, RBLoc,
                          diag::expected_rbrace_in_brace_stmt, LBLoc)) {
    // We recovered do don't propagate any error status (but still preserve
    // HasCodeCompletion).
    Status.clearIsError();
  }

  return makeParserResult(Status,
                          BraceStmt::create(Context, LBLoc, Entries, RBLoc));
}

static ParserStatus parseOptionalControlTransferTarget(Parser &P,
                                                       Identifier &Target,
                                                       SourceLoc &TargetLoc,
                                                       StmtKind Kind) {
  // If we have an identifier after 'break' or 'continue', which is not the
  // start of another stmt or decl, we assume it is the label to break to,
  // unless there is a line break.  There is ambiguity with expressions (e.g.
  // "break x+y") but since the expression after the them is dead, we don't feel
  // bad eagerly parsing this.
  if (!P.Tok.isAtStartOfLine()) {
    if (P.Tok.is(tok::identifier) && !P.isStartOfStmt(/*preferExpr*/ true) &&
        !P.isStartOfSwiftDecl()) {
      TargetLoc = P.consumeIdentifier(Target, /*diagnoseDollarPrefix=*/false);
      return makeParserSuccess();
    } else if (P.Tok.is(tok::code_complete)) {
      if (P.CodeCompletionCallbacks)
        P.CodeCompletionCallbacks->completeStmtLabel(Kind);
      TargetLoc = P.consumeToken(tok::code_complete);
      return makeParserCodeCompletionStatus();
    }
  }
  return makeParserSuccess();
}

/// parseStmtBreak
///
///   stmt-break:
///     'break' identifier?
///
ParserResult<Stmt> Parser::parseStmtBreak() {
  SourceLoc Loc = consumeToken(tok::kw_break);
  SourceLoc TargetLoc;
  Identifier Target;
  ParserStatus Status;
  Status |= parseOptionalControlTransferTarget(*this, Target, TargetLoc,
                                               StmtKind::Break);

  auto *BS = new (Context) BreakStmt(Loc, Target, TargetLoc, CurDeclContext);
  return makeParserResult(Status, BS);
}

/// parseStmtContinue
///
///   stmt-continue:
///     'continue' identifier?
///
ParserResult<Stmt> Parser::parseStmtContinue() {
  SourceLoc Loc = consumeToken(tok::kw_continue);
  SourceLoc TargetLoc;
  Identifier Target;
  ParserStatus Status;
  Status |= parseOptionalControlTransferTarget(*this, Target, TargetLoc,
                                               StmtKind::Continue);

  auto *CS = new (Context) ContinueStmt(Loc, Target, TargetLoc, CurDeclContext);
  return makeParserResult(Status, CS);
}


/// parseStmtReturn
///
///   stmt-return:
///     'return' expr?
///   
ParserResult<Stmt> Parser::parseStmtReturn(SourceLoc tryLoc) {
  SourceLoc ReturnLoc = consumeToken(tok::kw_return);

  if (Tok.is(tok::code_complete)) {
    auto CCE = new (Context) CodeCompletionExpr(Tok.getLoc());
    auto Result =
        makeParserResult(ReturnStmt::createParsed(Context, ReturnLoc, CCE));
    if (CodeCompletionCallbacks) {
      CodeCompletionCallbacks->completeReturnStmt(CCE);
    }
    Result.setHasCodeCompletionAndIsError();
    consumeToken();
    return Result;
  }

  auto isStartOfReturnExpr = [&]() {
    if (Tok.isAny(tok::r_brace, tok::semi, tok::eof, tok::pound_if,
                  tok::pound_error, tok::pound_warning, tok::pound_endif,
                  tok::pound_else, tok::pound_elseif)) {
      return false;
    }
    // Allowed for if/switch/do expressions.
    if (Tok.isAny(tok::kw_if, tok::kw_switch) ||
        (Tok.is(tok::kw_do) &&
         Context.LangOpts.hasFeature(Feature::DoExpressions))) {
      return true;
    }
    if (isStartOfStmt(/*preferExpr*/ true) || isStartOfSwiftDecl())
      return false;

    return true;
  };

  // Handle the ambiguity between consuming the expression and allowing the
  // enclosing stmt-brace to get it by eagerly eating it unless the return is
  // followed by a '}', ';', statement or decl start keyword sequence.
  if (isStartOfReturnExpr()) {
    SourceLoc ExprLoc = Tok.getLoc();

    // Issue a warning when the returned expression is on a different line than
    // the return keyword, but both have the same indentation.
    if (SourceMgr.getLineAndColumnInBuffer(ReturnLoc).second ==
        SourceMgr.getLineAndColumnInBuffer(ExprLoc).second) {
      diagnose(ExprLoc, diag::unindented_code_after_return);
      diagnose(ExprLoc, diag::indent_expression_to_silence);
    }

    ParserResult<Expr> Result = parseExpr(diag::expected_expr_return);
    if (Result.isNull()) {
      // Create an ErrorExpr to tell the type checker that this return
      // statement had an expression argument in the source.  This suppresses
      // the error about missing return value in a non-void function.
      Result = makeParserErrorResult(new (Context) ErrorExpr(ExprLoc));
    }

    if (tryLoc.isValid()) {
      diagnose(tryLoc, diag::try_must_come_after_stmt, /*return=*/0)
        .fixItInsert(ExprLoc, "try ")
        .fixItRemoveChars(tryLoc, ReturnLoc);

      // Note: We can't use tryLoc here because that's outside the ReturnStmt's
      // source range.
      if (Result.isNonNull() && !isa<ErrorExpr>(Result.get()))
        Result = makeParserResult(new (Context) TryExpr(ExprLoc, Result.get()));
    }

    return makeParserResult(
        Result,
        ReturnStmt::createParsed(Context, ReturnLoc, Result.getPtrOrNull()));
  }

  if (tryLoc.isValid())
    diagnose(tryLoc, diag::try_on_stmt, "return");

  return makeParserResult(
      ReturnStmt::createParsed(Context, ReturnLoc, nullptr));
}

/// parseStmtYield
///
///   stmt-yield:
///     'yield' expr
///     'yield' '(' expr-list ')'
///
/// Note that a parenthesis always starts the second (list) grammar.
ParserResult<Stmt> Parser::parseStmtYield(SourceLoc tryLoc) {
  SourceLoc yieldLoc = consumeToken(tok::kw_yield);

  if (Tok.is(tok::code_complete)) {
    auto cce = new (Context) CodeCompletionExpr(Tok.getLoc());
    auto result = makeParserResult(
      YieldStmt::create(Context, yieldLoc, SourceLoc(), cce, SourceLoc()));
    if (CodeCompletionCallbacks) {
      CodeCompletionCallbacks->completeYieldStmt(cce, /*index=*/std::nullopt);
    }
    result.setHasCodeCompletionAndIsError();
    consumeToken();
    return result;
  }

  ParserStatus status;
  SourceLoc lpLoc, rpLoc;
  SmallVector<Expr*, 4> yields;
  if (Tok.is(tok::l_paren)) {
    // If there was a 'try' on the yield, and there are multiple
    // yielded values, suggest just removing the try instead of
    // suggesting adding it to every yielded value.
    if (tryLoc.isValid()) {
      diagnose(tryLoc, diag::try_must_come_after_stmt, /*yield=*/2)
        .fixItRemoveChars(tryLoc, yieldLoc);
    }

    SmallVector<ExprListElt, 4> yieldElts;
    status = parseExprList(tok::l_paren, tok::r_paren, /*isArgumentList*/ false,
                           lpLoc, yieldElts, rpLoc);
    for (auto &elt : yieldElts) {
      // We don't accept labels in a list of yields.
      if (elt.LabelLoc.isValid()) {
        diagnose(elt.LabelLoc, diag::unexpected_label_yield)
          .fixItRemoveChars(elt.LabelLoc, elt.E->getStartLoc());
      }
      yields.push_back(elt.E);
    }
  } else {
    SourceLoc beginLoc = Tok.getLoc();

    // There's a single yielded value, so suggest moving 'try' before it.
    if (tryLoc.isValid()) {
      diagnose(tryLoc, diag::try_must_come_after_stmt, /*yield=*/2)
        .fixItInsert(beginLoc, "try ")
        .fixItRemoveChars(tryLoc, yieldLoc);
    }

    auto expr = parseExpr(diag::expected_expr_yield);
    if (expr.hasCodeCompletion() && expr.isNonNull()) {
      status |= expr;
      yields.push_back(expr.get());
    } else if (expr.isParseErrorOrHasCompletion()) {
      auto endLoc = (Tok.getLoc() == beginLoc ? beginLoc : PreviousLoc);
      yields.push_back(
        new (Context) ErrorExpr(SourceRange(beginLoc, endLoc)));
    } else {
      yields.push_back(expr.get());
    }
  }

  return makeParserResult(
           status, YieldStmt::create(Context, yieldLoc, lpLoc, yields, rpLoc));
}

bool Parser::isContextualThenKeyword(bool preferExpr) {
  if (!Context.LangOpts.hasFeature(Feature::ThenStatements))
    return false;

  if (!Tok.isContextualKeyword("then"))
    return false;

  // If we want to prefer an expr, and aren't at the start of a newline, then
  // don't parse a ThenStmt.
  if (preferExpr && !Tok.isAtStartOfLine())
    return false;

  // 'then' immediately followed by '('/'[' is a function/subscript call. If
  // immediately followed by '.', it's a member access.
  if (peekToken().isAny(tok::l_paren, tok::l_square, tok::period)) {
    auto tokEndLoc = Lexer::getLocForEndOfToken(SourceMgr, Tok.getLoc());
    return peekToken().getLoc() != tokEndLoc;
  }

  // 'then' followed by '{' is a trailing closure on a function call.
  if (peekToken().is(tok::l_brace))
    return false;

  // If we have 'then' followed by an infix or postfix operator, we know this
  // must be an expression.
  if (peekToken().isBinaryOperatorLike() || peekToken().isPostfixOperatorLike())
    return false;

  // These act like binary operators.
  if (peekToken().isAny(tok::kw_is, tok::kw_as))
    return false;

  return true;
}

/// parseStmtThen
///
/// stmt-then:
///   'then' expr
///
ParserResult<Stmt> Parser::parseStmtThen(SourceLoc tryLoc) {
  SourceLoc thenLoc = consumeToken(tok::kw_then);

  if (Tok.is(tok::code_complete)) {
    auto ccLoc = consumeToken();
    auto *CCE = new (Context) CodeCompletionExpr(ccLoc);
    if (CodeCompletionCallbacks)
      CodeCompletionCallbacks->completeThenStmt(CCE);

    return makeParserCodeCompletionResult(
        ThenStmt::createParsed(Context, thenLoc, CCE));
  }

  auto exprLoc = Tok.getLoc();

  // Issue a warning when the expression is on a different line than
  // the 'then' keyword, but both have the same indentation.
  if (SourceMgr.getLineAndColumnInBuffer(thenLoc).second ==
      SourceMgr.getLineAndColumnInBuffer(exprLoc).second) {
    diagnose(exprLoc, diag::unindented_code_after_then);
    diagnose(exprLoc, diag::indent_expression_to_silence);
  }

  ParserResult<Expr> result = parseExpr(diag::expected_expr_after_then);
  bool hasCodeCompletion = result.hasCodeCompletion();

  // If we couldn't parse an expr, fill it the gap with an ErrorExpr, as
  // ThenStmt expects an expression node.
  if (result.isNull())
    result = makeParserErrorResult(new (Context) ErrorExpr(exprLoc));

  if (tryLoc.isValid()) {
    diagnose(tryLoc, diag::try_must_come_after_stmt, /*then=*/3)
        .fixItInsert(exprLoc, "try ")
        .fixItRemoveChars(tryLoc, thenLoc);

    // Note: We can't use tryLoc here because that's outside the ThenStmt's
    // source range.
    if (!isa<ErrorExpr>(result.get()))
      result = makeParserResult(new (Context) TryExpr(exprLoc, result.get()));
  }

  if (hasCodeCompletion)
    result.setHasCodeCompletionAndIsError();

  return makeParserResult(
      result, ThenStmt::createParsed(Context, thenLoc, result.get()));
}

/// parseStmtThrow
///
/// stmt-throw
///   'throw' expr
///
ParserResult<Stmt> Parser::parseStmtThrow(SourceLoc tryLoc) {
  SourceLoc throwLoc = consumeToken(tok::kw_throw);
  SourceLoc exprLoc;
  if (Tok.isNot(tok::eof))
    exprLoc = Tok.getLoc();

  ParserResult<Expr> Result = parseExpr(diag::expected_expr_throw);
  bool hasCodeCompletion = Result.hasCodeCompletion();

  if (Result.isNull())
    Result = makeParserErrorResult(new (Context) ErrorExpr(throwLoc));

  if (tryLoc.isValid() && exprLoc.isValid()) {
    diagnose(tryLoc, diag::try_must_come_after_stmt, /*throw=*/1)
      .fixItInsert(exprLoc, "try ")
      .fixItRemoveChars(tryLoc, throwLoc);

    // Note: We can't use tryLoc here because that's outside the ThrowStmt's
    // source range.
    if (Result.isNonNull() && !isa<ErrorExpr>(Result.get()))
      Result = makeParserResult(new (Context) TryExpr(exprLoc, Result.get()));
  }

  if (hasCodeCompletion)
    Result.setHasCodeCompletionAndIsError();

  return makeParserResult(Result,
              new (Context) ThrowStmt(throwLoc, Result.get()));
}

/// parseStmtDiscard
///
/// stmt-discard
///   'discard' 'self'
///
ParserResult<Stmt> Parser::parseStmtDiscard() {
  SourceLoc discardLoc = consumeToken(tok::kw_discard);

  SourceLoc exprLoc;
  if (Tok.isNot(tok::eof))
    exprLoc = Tok.getLoc();

  // We parse the whole expression, because we might have something like:
  //                         discard self.x.y
  // and we want to emit good diagnostics for this later on.
  ParserResult<Expr> Result = parseExpr(diag::expected_expr_discard);
  bool hasCodeCompletion = Result.hasCodeCompletion();

  if (Result.isNull())
    Result = makeParserErrorResult(new (Context) ErrorExpr(discardLoc));

  if (hasCodeCompletion)
    Result.setHasCodeCompletionAndIsError();

  return makeParserResult(Result,
                          new (Context) DiscardStmt(discardLoc, Result.get()));
}

/// parseStmtDefer
///
///   stmt-defer:
///     'defer' brace-stmt
///
ParserResult<Stmt> Parser::parseStmtDefer() {
  SourceLoc DeferLoc = consumeToken(tok::kw_defer);

  // Expand out the defer into a closure and call, which we can typecheck
  // and emit where needed.
  //
  // The AST representation for a defer statement is a bit weird.  We retain the
  // brace statement that the user wrote, but actually model this as if they
  // wrote:
  //
  //    func tmpClosure() { body }
  //    tmpClosure()   // This is emitted on each path that needs to run this.
  //
  // As such, the body of the 'defer' is actually type checked within the
  // closure's DeclContext.
  auto *DS = DeferStmt::create(CurDeclContext, DeferLoc);
  ParserStatus Status;
  {
    auto *tempDecl = DS->getTempDecl();
    // Change the DeclContext for any variables declared in the defer to be within
    // the defer closure.
    ParseFunctionBody cc(*this, tempDecl);
    llvm::SaveAndRestore<std::optional<StableHasher>> T(
        CurrentTokenHash, StableHasher::defaultHasher());

    ParserResult<BraceStmt> Body =
      parseBraceItemList(diag::expected_lbrace_after_defer);
    if (Body.isNull())
      return nullptr;
    Status |= Body;

    // Clone the current hasher and extract a Fingerprint.
    StableHasher currentHash{*CurrentTokenHash};
    Fingerprint fp(std::move(currentHash));
    tempDecl->setBodyParsed(Body.get(), fp);
  }

  return makeParserResult(Status, DS);
}

namespace {
  struct GuardedPattern {
    Pattern *ThePattern = nullptr;
    SourceLoc WhereLoc;
    Expr *Guard = nullptr;
  };
  
  /// Contexts in which a guarded pattern can appear.
  enum class GuardedPatternContext {
    Case,
    Catch,
  };
} // unnamed namespace

static void parseWhereGuard(Parser &P, GuardedPattern &result,
                            ParserStatus &status,
                            GuardedPatternContext parsingContext,
                            bool isExprBasic) {
  if (P.Tok.is(tok::kw_where)) {
    result.WhereLoc = P.consumeToken(tok::kw_where);
    SourceLoc startOfGuard = P.Tok.getLoc();

    auto diagKind = [=]() -> Diag<> {
      switch (parsingContext) {
      case GuardedPatternContext::Case:
        return diag::expected_case_where_expr;
      case GuardedPatternContext::Catch:
        return diag::expected_catch_where_expr;
      }
      llvm_unreachable("bad context");
    }();
    ParserResult<Expr> guardResult = P.parseExprImpl(diagKind, isExprBasic);
    status |= guardResult;

    // Use the parsed guard expression if possible.
    if (guardResult.isNonNull()) {
      result.Guard = guardResult.get();

      // Otherwise, fake up an ErrorExpr.
    } else {
      // If we didn't consume any tokens failing to parse the
      // expression, don't put in the source range of the ErrorExpr.
      SourceRange errorRange;
      if (startOfGuard == P.Tok.getLoc()) {
        errorRange = result.WhereLoc;
      } else {
        errorRange = SourceRange(startOfGuard, P.PreviousLoc);
      }
      result.Guard = new (P.Context) ErrorExpr(errorRange);
    }
  }
}

/// Parse a pattern-matching clause for a case or catch statement,
/// including the guard expression:
///
///    pattern 'where' expr
static void parseGuardedPattern(Parser &P, GuardedPattern &result,
                                ParserStatus &status,
                                SmallVectorImpl<VarDecl *> &boundDecls,
                                GuardedPatternContext parsingContext,
                                bool isFirstPattern) {
  ParserResult<Pattern> patternResult;

  bool isExprBasic = [&]() -> bool {
    switch (parsingContext) {
    // 'case' is terminated with a colon and so allows a trailing closure.
    case GuardedPatternContext::Case:
      return false;
    // 'catch' is terminated with a brace and so cannot.
    case GuardedPatternContext::Catch:
      return true;
    }
    llvm_unreachable("bad pattern context");
  }();

  // Do some special-case code completion for the start of the pattern.
  if (P.Tok.is(tok::code_complete)) {
    auto CCE = new (P.Context) CodeCompletionExpr(P.Tok.getLoc());
    result.ThePattern =
        ExprPattern::createParsed(P.Context, CCE, P.CurDeclContext);
    if (P.CodeCompletionCallbacks) {
      switch (parsingContext) {
      case GuardedPatternContext::Case:
        P.CodeCompletionCallbacks->completeCaseStmtBeginning(CCE);
        break;
      case GuardedPatternContext::Catch:
        P.CodeCompletionCallbacks->completePostfixExprBeginning(CCE);
        break;
      }
    }
    P.consumeToken(tok::code_complete);
    status.setHasCodeCompletionAndIsError();
    return;
  }

  // If this is a 'catch' clause and we have "catch {" or "catch where...",
  // then we get an implicit "let error" pattern.
  if (parsingContext == GuardedPatternContext::Catch &&
      P.Tok.isAny(tok::l_brace, tok::kw_where)) {
    patternResult = makeParserResult(
        BindingPattern::createImplicitCatch(P.CurDeclContext, P.Tok.getLoc()));
  }

  // Okay, if the special code-completion didn't kick in, parse a
  // matching pattern.
  if (patternResult.isNull()) {
    llvm::SaveAndRestore<decltype(P.InBindingPattern)> T(
        P.InBindingPattern, PatternBindingState::InMatchingPattern);
    patternResult = P.parseMatchingPattern(isExprBasic);
  }

  // If that didn't work, use a bogus pattern so that we can fill out
  // the AST.
  if (patternResult.isNull()) {
    auto *AP = new (P.Context) AnyPattern(P.PreviousLoc);
    if (P.PreviousLoc.isInvalid())
      AP->setImplicit();
    patternResult = makeParserErrorResult(AP);
  }

  // Fill in the pattern.
  status |= patternResult;
  result.ThePattern = patternResult.get();

  if (isFirstPattern) {
    // Add variable bindings from the pattern to the case scope.  We have
    // to do this with a full AST walk, because the freshly parsed pattern
    // represents tuples and var patterns as tupleexprs and
    // unresolved_pattern_expr nodes, instead of as proper pattern nodes.
    patternResult.get()->forEachVariable([&](VarDecl *VD) {
      boundDecls.push_back(VD);
    });

    // Parse the optional 'where' guard.
    parseWhereGuard(P, result, status, parsingContext, isExprBasic);
  } else {
    // If boundDecls already contains variables, then we must match the
    // same number and same names in this pattern as were declared in a
    // previous pattern (and later we will make sure they have the same
    // types).
    SmallVector<VarDecl*, 4> repeatedDecls;
    patternResult.get()->forEachVariable([&](VarDecl *VD) {
      if (!VD->hasName())
        return;
      
      bool found = false;
      for (auto previous : boundDecls) {
        if (previous->hasName() && previous->getName() == VD->getName()) {
          found = true;
          break;
        }
      }
      if (!found) {
        // Diagnose a declaration that doesn't match a previous pattern.
        P.diagnose(VD->getLoc(), diag::extra_var_in_multiple_pattern_list, VD->getName());
        status.setIsParseError();
      }
      repeatedDecls.push_back(VD);
    });
    
    for (auto previous : boundDecls) {
      bool found = false;
      for (auto repeat : repeatedDecls) {
        if (previous->hasName() && previous->getName() == repeat->getName()) {
          found = true;
          break;
        }
      }
      if (!found) {
        // Diagnose a previous declaration that is missing in this pattern.
        P.diagnose(previous->getLoc(), diag::extra_var_in_multiple_pattern_list, previous->getName());
        status.setIsParseError();
      }
    }

    // Parse the optional 'where' guard, with this particular pattern's bound
    // vars in scope.
    parseWhereGuard(P, result, status, parsingContext, isExprBasic);
  }
}

/// Validate availability spec list, emitting diagnostics if necessary and
/// removing specs for unrecognized platforms.
static void
validateAvailabilitySpecList(Parser &P,
                             const SmallVectorImpl<AvailabilitySpec *> &Specs,
                             Parser::AvailabilitySpecSource Source) {
  std::optional<SourceLoc> WildcardSpecLoc = std::nullopt;

  if (Specs.size() == 1) {
    // @available(swift N) and @available(_PackageDescription N) are allowed
    // only in isolation; they cannot be combined with other availability specs
    // in a single list.
    auto domain = Specs[0]->getDomain();
    if (domain && !domain->isPlatform())
      return;
  }

  for (auto *Spec : Specs) {
    if (Spec->isWildcard()) {
      WildcardSpecLoc = Spec->getStartLoc();
    }
  }

  switch (Source) {
  case Parser::AvailabilitySpecSource::Available: {
    if (WildcardSpecLoc == std::nullopt) {
      SourceLoc InsertWildcardLoc = P.PreviousLoc;
      P.diagnose(InsertWildcardLoc, diag::availability_query_wildcard_required)
        .fixItInsertAfter(InsertWildcardLoc, ", *");
    }
    break;
  }
  case Parser::AvailabilitySpecSource::Unavailable: {
    if (WildcardSpecLoc != std::nullopt) {
      SourceLoc Loc = WildcardSpecLoc.value();
      P.diagnose(Loc, diag::unavailability_query_wildcard_not_required)
        .fixItRemove(Loc);
    }
    break;
  }
  case Parser::AvailabilitySpecSource::Macro: {
    if (WildcardSpecLoc != std::nullopt) {
      SourceLoc Loc = WildcardSpecLoc.value();
      P.diagnose(Loc, diag::attr_availability_wildcard_in_macro);
    }
    break;
  }
  }
}

// #available(...)
// #unavailable(...)
ParserResult<PoundAvailableInfo> Parser::parseStmtConditionPoundAvailable() {
  tok MainToken;
  AvailabilitySpecSource Source;
  bool isUnavailability;
  if (Tok.is(tok::pound_available)) {
    MainToken = tok::pound_available;
    Source = AvailabilitySpecSource::Available;
    isUnavailability = false;
  } else {
    MainToken = tok::pound_unavailable;
    Source = AvailabilitySpecSource::Unavailable;
    isUnavailability = true;
  }

  SourceLoc PoundLoc;

  PoundLoc = consumeToken(MainToken);

  if (!Tok.isFollowingLParen()) {
    diagnose(Tok, diag::avail_query_expected_condition);
    return makeParserError();
  }

  StructureMarkerRAII ParsingAvailabilitySpecList(*this, Tok);

  SourceLoc LParenLoc = consumeToken(tok::l_paren);

  SmallVector<AvailabilitySpec *, 5> Specs;
  ParserStatus Status = parseAvailabilitySpecList(Specs, Source);

  SourceLoc RParenLoc;
  if (parseMatchingToken(tok::r_paren, RParenLoc,
                         diag::avail_query_expected_rparen, LParenLoc))
    Status.setIsParseError();

  // Diagnose #available == false as being a wrong spelling of #unavailable.
  if (!isUnavailability && Tok.isAnyOperator() && Tok.getText() == "==" &&
      peekToken().is(tok::kw_false)) {
    diagnose(Tok, diag::false_available_is_called_unavailable)
      .highlight(SourceRange(PoundLoc, peekToken().getLoc()))
      .fixItReplace(PoundLoc, getTokenText(tok::pound_unavailable))
      .fixItRemove(SourceRange(Tok.getLoc(), peekToken().getLoc()));
    consumeToken();
    consumeToken();
    Status.setIsParseError();
  }

  auto *result = PoundAvailableInfo::create(Context, PoundLoc, LParenLoc, Specs,
                                            RParenLoc, isUnavailability);
  return makeParserResult(Status, result);
}

ParserStatus Parser::parseAvailabilityMacroDefinition(
    std::string &Name, llvm::VersionTuple &Version,
    SmallVectorImpl<AvailabilitySpec *> &Specs) {

  // Prime the lexer.
  if (Tok.is(tok::NUM_TOKENS))
    consumeTokenWithoutFeedingReceiver();

  if (!Tok.isIdentifierOrUnderscore()) {
    diagnose(Tok, diag::attr_availability_missing_macro_name);
    return makeParserError();
  }

  Name = Tok.getText();
  consumeToken();

  if (Tok.isAny(tok::integer_literal, tok::floating_literal)) {
    SourceRange VersionRange;
    if (parseVersionTuple(Version, VersionRange,
                          diag::avail_query_expected_version_number)) {
      return makeParserError();
    }
  }

  if (!consumeIf(tok::colon)) {
    diagnose(Tok, diag::attr_availability_expected_colon_macro, Name);
    return makeParserError();
  }

  return parseAvailabilitySpecList(Specs, AvailabilitySpecSource::Macro);
}

ParserStatus
Parser::parseAvailabilitySpecList(SmallVectorImpl<AvailabilitySpec *> &Specs,
                                  AvailabilitySpecSource Source) {
  ParserStatus Status = makeParserSuccess();

  // We don't use parseList() because we want to provide more specific
  // diagnostics disallowing operators in version specs.
  while (1) {
    // First look for a macro as we need Specs for the expansion.
    bool MatchedAMacro = false;
    switch (Source) {
    case AvailabilitySpecSource::Available:
    case AvailabilitySpecSource::Unavailable:
      if (Tok.is(tok::identifier)) {
        SmallVector<AvailabilitySpec *, 4> MacroSpecs;
        ParserStatus MacroStatus = parseAvailabilityMacro(MacroSpecs);

        if (MacroStatus.isError()) {
          // There's a parsing error if the platform name matches a macro
          // but something goes wrong after.
          Status.setIsParseError();
          MatchedAMacro = true;
        } else {
          MatchedAMacro = !MacroSpecs.empty();
          Specs.append(MacroSpecs.begin(), MacroSpecs.end());
        }
      }
      break;
    case AvailabilitySpecSource::Macro: 
      break;
    }

    if (!MatchedAMacro) {
      auto SpecResult = parseAvailabilitySpec();
      if (auto *Spec = SpecResult.getPtrOrNull()) {
        Specs.push_back(Spec);
      } else {
        if (SpecResult.hasCodeCompletion()) {
          return makeParserCodeCompletionStatus();
        }
        Status.setIsParseError();
      }
    }

    // We don't allow binary operators to combine specs.
    if (Tok.isBinaryOperator()) {
      diagnose(Tok, diag::avail_query_disallowed_operator, Tok.getText());
      consumeToken();
      Status.setIsParseError();
    } else if (consumeIf(tok::comma)) {
      // End of unavailable spec list with a trailing comma.
      if (Source == AvailabilitySpecSource::Unavailable && Tok.is(tok::r_paren)) {
        break;
      }
      // There is more to parse in this list.

      // Before continuing to parse the next specification, we check that it's
      // also in the shorthand syntax and provide a more specific diagnostic if
      // that's not the case.
      if (Tok.isIdentifierOrUnderscore() &&
          !peekToken().isAny(tok::integer_literal, tok::floating_literal) &&
          !Specs.empty()) {
        auto Text = Tok.getText();
        if (Text == "deprecated" || Text == "renamed" || Text == "introduced" ||
            Text == "message" || Text == "obsoleted" || Text == "unavailable") {
          auto *Previous = Specs.back();
          auto &SourceManager = Context.SourceMgr;
          auto PreviousSpecText =
              SourceManager.extractText(L->getCharSourceRangeFromSourceRange(
                  SourceManager, Previous->getSourceRange()));

          diagnose(Tok,
                   diag::avail_query_argument_and_shorthand_mix_not_allowed,
                   Text, PreviousSpecText);

          // If this was preceded by a single platform version constraint, we
          // can guess that the intention was to treat it as 'introduced' and
          // suggest a fix-it to combine them.
          if (Specs.size() == 1 &&
              Previous->getPlatform() != PlatformKind::none &&
              Text != "introduced") {
            auto PlatformNameEndLoc = Lexer::getLocForEndOfToken(
                SourceManager, Previous->getStartLoc());

            diagnose(Previous->getStartLoc(),
                     diag::avail_query_meant_introduced)
                .fixItInsert(PlatformNameEndLoc, ", introduced:");
          }

          Status.setIsParseError();
          break;
        }
      }

      // Otherwise, keep going.
    } else {
      break;
    }
  }

  if (Status.isSuccess() && !Status.hasCodeCompletion())
    validateAvailabilitySpecList(*this, Specs, Source);

  return Status;
}

// #_hasSymbol(...)
ParserResult<PoundHasSymbolInfo> Parser::parseStmtConditionPoundHasSymbol() {
  SourceLoc PoundLoc = consumeToken(tok::pound__hasSymbol);

  if (!Tok.isFollowingLParen()) {
    diagnose(Tok, diag::has_symbol_expected_lparen);
    return makeParserError();
  }

  SourceLoc LParenLoc = consumeToken(tok::l_paren);
  ParserStatus status = makeParserSuccess();

  auto ExprResult = parseExprBasic(diag::has_symbol_expected_expr);
  status |= ExprResult;

  SourceLoc RParenLoc;
  if (parseMatchingToken(tok::r_paren, RParenLoc,
                         diag::has_symbol_expected_rparen, LParenLoc))
    status.setIsParseError();

  auto *result = PoundHasSymbolInfo::create(
      Context, PoundLoc, LParenLoc, ExprResult.getPtrOrNull(), RParenLoc);
  return makeParserResult(status, result);
}

ParserStatus
Parser::parseStmtConditionElement(SmallVectorImpl<StmtConditionElement> &result,
                                  Diag<> DefaultID, StmtKind ParentKind,
                                  StringRef &BindingKindStr) {
  ParserStatus Status;

  // Diagnose !#available as being a wrong spelling of #unavailable.
  if (Tok.getText() == "!" && peekToken().is(tok::pound_available)) {
    SourceRange Range = SourceRange(Tok.getLoc(), peekToken().getLoc());
    diagnose(Tok, diag::false_available_is_called_unavailable)
      .fixItReplace(Range, getTokenText(tok::pound_unavailable));
    // For better error recovery, reject but allow parsing to continue.
    consumeToken();
  }

  // Parse a leading #available/#unavailable condition if present.
  if (Tok.isAny(tok::pound_available, tok::pound_unavailable)) {
    auto res = parseStmtConditionPoundAvailable();
    if (res.isNull() || res.hasCodeCompletion()) {
      Status |= res;
      return Status;
    }
    BindingKindStr = StringRef();
    result.push_back({res.get()});
    return Status;
  }

  // Parse a leading #_hasSymbol condition if present.
  if (Tok.is(tok::pound__hasSymbol)) {
    auto res = parseStmtConditionPoundHasSymbol();
    if (res.isNull() || res.hasCodeCompletion()) {
      Status |= res;
      return Status;
    }
    BindingKindStr = StringRef();
    result.push_back({res.get()});
    return Status;
  }

  // Handle code completion after the #.
  if (Tok.is(tok::pound) && peekToken().is(tok::code_complete) &&
      Tok.getLoc().getAdvancedLoc(1) == peekToken().getLoc()) {
    auto Expr = parseExprPoundCodeCompletion(ParentKind);
    Status |= Expr;
    result.push_back(Expr.get());
  }

  // Parse the basic expression case.  If we have a leading let/var/case
  // keyword or an assignment, then we know this is a binding.
  if (Tok.isNot(tok::kw_let, tok::kw_var, tok::kw_case) &&
      (!Context.LangOpts.hasFeature(Feature::ReferenceBindings) ||
       Tok.isNot(tok::kw_inout))) {
    // If we lack it, then this is theoretically a boolean condition.
    // However, we also need to handle migrating from Swift 2 syntax, in
    // which a comma followed by an expression could actually be a pattern
    // clause followed by a binding.  Determine what we have by checking for a
    // syntactically valid pattern followed by an '=', which can never be a
    // boolean condition.
    //
    // However, if this is the first clause, and we see "x = y", then this is
    // almost certainly a typo for '==' and definitely not a continuation of
    // another clause, so parse it as an expression.  This also avoids
    // lookahead + backtracking on simple if conditions that are obviously
    // boolean conditions.
    auto isBooleanExpr = [&]() -> bool {
      Parser::BacktrackingScope Backtrack(*this);
      return !canParseTypedPattern() || Tok.isNot(tok::equal);
    };

    if (BindingKindStr.empty() || isBooleanExpr()) {
      auto diagID = result.empty() ? DefaultID :
        diag::expected_expr_conditional;
      auto BoolExpr = parseExprBasic(diagID);
      Status |= BoolExpr;
      if (BoolExpr.isNull())
        return Status;
      result.push_back(BoolExpr.get());
      BindingKindStr = StringRef();
      return Status;
    }
  }

  SourceLoc IntroducerLoc;
  if (Tok.isAny(tok::kw_let, tok::kw_var, tok::kw_case) ||
      (Context.LangOpts.hasFeature(Feature::ReferenceBindings) &&
       Tok.isAny(tok::kw_inout))) {
    BindingKindStr = Tok.getText();
    IntroducerLoc = consumeToken();
  } else {
    // If we lack the leading let/var/case keyword, then we're here because
    // the user wrote something like "if let x = foo(), y = bar() {".  Fix
    // this by inserting a new 'let' keyword before y.
    IntroducerLoc = Tok.getLoc();
    assert(!BindingKindStr.empty() &&
            "Shouldn't get here without a leading binding");
    diagnose(Tok.getLoc(), diag::expected_binding_keyword, BindingKindStr)
      .fixItInsert(Tok.getLoc(), BindingKindStr.str()+" ");
  }

  // We're parsing a conditional binding.
  assert(CurDeclContext->isLocalContext() &&
          "conditional binding in non-local context?!");
    
  ParserResult<Pattern> ThePattern;
    
  if (BindingKindStr == "case") {
    // In our recursive parse, remember that we're in a matching pattern.
    llvm::SaveAndRestore<decltype(InBindingPattern)> T(
        InBindingPattern, PatternBindingState::InMatchingPattern);

    // Reset async attribute in parser context.
    llvm::SaveAndRestore<bool> AsyncAttr(InPatternWithAsyncAttribute, false);

    ThePattern = parseMatchingPattern(/*isExprBasic*/ true);
  } else if (Tok.is(tok::kw_case)) {
    // If will probably be a common typo to write "if let case" instead of
    // "if case let" so detect this and produce a nice fixit.
    diagnose(IntroducerLoc, diag::wrong_condition_case_location,
              BindingKindStr)
    .fixItRemove(IntroducerLoc)
    .fixItInsertAfter(Tok.getLoc(), " " + BindingKindStr.str());

    consumeToken(tok::kw_case);

    auto newPatternBindingState = PatternBindingState::get(BindingKindStr)
      .value_or(PatternBindingState(PatternBindingState::InVar));
    BindingKindStr = "case";
    
    // In our recursive parse, remember that we're in a var/let pattern.
    llvm::SaveAndRestore<decltype(InBindingPattern)> T(InBindingPattern,
                                                       newPatternBindingState);

    // Reset async attribute in parser context.
    llvm::SaveAndRestore<bool> AsyncAttr(InPatternWithAsyncAttribute, false);

    ThePattern = parseMatchingPattern(/*isExprBasic*/ true);
    
    if (ThePattern.isNonNull()) {
      auto *P = new (Context)
          BindingPattern(IntroducerLoc, *newPatternBindingState.getIntroducer(),
                         ThePattern.get());
      ThePattern = makeParserResult(Status, P);
    }

  } else if (Tok.is(tok::code_complete)) {
    if (CodeCompletionCallbacks) {
      CodeCompletionCallbacks->completeOptionalBinding();
    }
    ThePattern = makeParserResult(new (Context) AnyPattern(Tok.getLoc()));
    ThePattern.setHasCodeCompletionAndIsError();
    consumeToken(tok::code_complete);
  } else {
    // Otherwise, this is an implicit optional binding "if let".
    ThePattern = parseMatchingPatternAsBinding(
        PatternBindingState::get(BindingKindStr)
            .value_or(PatternBindingState(PatternBindingState::InVar)),
        IntroducerLoc,
        /*isExprBasic*/ true);
    // The let/var pattern is part of the statement.
    if (Pattern *P = ThePattern.getPtrOrNull())
      P->setImplicit();
  }

  ThePattern = parseOptionalPatternTypeAnnotation(ThePattern);
  if (ThePattern.hasCodeCompletion()) {
    Status.setHasCodeCompletionAndIsError();

    // Skip to '=' so that the completion can see the expected type of the
    // pattern which is determined by the initializer. 
    skipUntilDeclStmtRBrace(tok::equal, tok::l_brace);
  }
    
  if (ThePattern.isNull()) {
    // Recover by creating AnyPattern.
    auto *AP = new (Context) AnyPattern(PreviousLoc);
    if (PreviousLoc.isInvalid())
      AP->setImplicit();
    ThePattern = makeParserResult(AP);
  }

  // Conditional bindings can have the format:
  //  `let newBinding = <expr>`, or
  //  `let newBinding`, which is shorthand for `let newBinding = newBinding`
  ParserResult<Expr> Init;
  if (Tok.is(tok::equal)) {
    consumeToken();
    Init = parseExprBasic(diag::expected_expr_conditional_var);
  } else if (!ThePattern.getPtrOrNull()->getBoundName().empty()) {
    auto bindingName = DeclNameRef(ThePattern.getPtrOrNull()->getBoundName());
    auto loc = DeclNameLoc(ThePattern.getPtrOrNull()->getEndLoc());
    auto declRefExpr = new (Context) UnresolvedDeclRefExpr(bindingName,
                                                           DeclRefKind::Ordinary,
                                                           loc);
    // We do NOT mark this declRefExpr as implicit because that'd avoid
    // reporting errors if it were used in a synchronous context in
    // 'diagnoseUnhandledAsyncSite'. There may be other ways to resolve the
    // reporting issue that we could explore in the future.
    //
    // Even though implicit, the ast node has correct source location.
    Init = makeParserResult(declRefExpr);
  } else if (BindingKindStr != "case") {
    // If the pattern is present but isn't an identifier, the user wrote
    // something invalid like `let foo.bar`. Emit a special diagnostic for this,
    // with a fix-it prepending "<#identifier#> = "
    //  - We don't emit this fix-it if the user wrote `case let` (etc),
    //    since the shorthand syntax isn't available for pattern matching
    auto diagLoc = ThePattern.get()->getSemanticsProvidingPattern()->getStartLoc();
    diagnose(diagLoc, diag::conditional_var_valid_identifiers_only)
      .fixItInsert(diagLoc, "<#identifier#> = ");

    // For better recovery, assume the expression pattern as the initializer,
    // and synthesize an optional AnyPattern.
    auto *semanticPattern = ThePattern.get()->getSemanticsProvidingPattern();
    if (auto *EP = dyn_cast<ExprPattern>(semanticPattern)) {
      Init = makeParserResult(EP->getSubExpr());
      auto *AP = AnyPattern::createImplicit(Context);
      ThePattern =
          makeParserResult(OptionalSomePattern::createImplicit(Context, AP));
    }
  } else {
    diagnose(Tok, diag::conditional_var_initializer_required);
  }

  if (Init.hasCodeCompletion())
    Status.setHasCodeCompletionAndIsError();

  if (Init.isNull()) {
    // Recover by creating ErrorExpr.
    Init = makeParserResult(new (Context)
                                ErrorExpr(ThePattern.get()->getEndLoc()));
  }

  result.push_back(ConditionalPatternBindingInfo::create(
      Context, IntroducerLoc, ThePattern.get(), Init.get()));
  return Status;
}

/// Returns `true` if the current token represents the start of a conditional
/// statement body.
bool Parser::isStartOfConditionalStmtBody() {
  if (!Tok.is(tok::l_brace)) {
    // Statement bodies always start with a '{'. If there is no '{', we can't be
    // at the statement body.
    return false;
  }

  Parser::BacktrackingScope Backtrack(*this);

  skipSingle();

  if (Tok.is(tok::eof)) {
    // There's nothing else in the source file that could be the statement body,
    // so this must be it.
    return true;
  }

  if (Tok.is(tok::semi)) {
    // We can't have a semicolon between the condition and the statement body,
    // so this must be the statement body.
    return true;
  }

  if (Tok.is(tok::kw_else)) {
    // If the current token is an `else` keyword, this must be the statement
    // body of an `if` statement since conditions can't be followed by `else`.
    return true;
  }

  if (Tok.is(tok::r_brace) || Tok.is(tok::r_paren)) {
    // A right brace or parenthesis cannot start a statement body, nor can the
    // condition list continue afterwards. So, this must be the statement body.
    // This covers cases like `if true, { if true, { } }` or `( if true, {
    // print(0) } )`. While the latter is not valid code, it improves
    // diagnostics.
    return true;
  }

  if (Tok.isAtStartOfLine()) {
    // If the current token is at the start of a line, it is most likely a
    // statement body. The only exceptions are:
    if (Tok.is(tok::comma)) {
      // If newline begins with ',' it must be a condition trailing comma, so
      // this can't be the statement body, e.g. if true, { true } , true {
      // print("body") }
      return false;
    }
    if (Tok.is(tok::oper_binary_spaced) || Tok.is(tok::oper_binary_unspaced)) {
      // If current token is a binary operator this can't be the statement body
      // since an `if` expression can't be the left-hand side of an operator,
      // e.g. if true, { true }
      // != nil
      // {
      //   print("body")
      // }
      return false;
    }
    // Excluded the above exceptions, this must be the statement body.
    return true;
  } else {
    // If the current token isn't at the start of a line and isn't `EOF`, `;`,
    // `else`, `)` or `}` this can't be the statement body.
    return false;
  }
}

/// Parse the condition of an 'if' or 'while'.
///
///   condition:
///     condition-clause (',' condition-clause)*
///   condition-clause:
///     expr-basic
///     ('var' | 'let' | 'case') pattern '=' expr-basic
///     '#available' '(' availability-spec (',' availability-spec)* ')'
///     '#_hasSymbol' '(' expr ')'
///
/// The use of expr-basic here disallows trailing closures, which are
/// problematic given the curly braces around the if/while body.
///
ParserStatus Parser::parseStmtCondition(StmtCondition &Condition,
                                        Diag<> DefaultID, StmtKind ParentKind) {
  ParserStatus Status;
  Condition = StmtCondition();

  SmallVector<StmtConditionElement, 4> result;

  // For error recovery purposes, keep track of the disposition of the last
  // pattern binding we saw ('let', 'var', or 'case').
  StringRef BindingKindStr;
  
  // We have a simple comma separated list of clauses, but also need to handle
  // a variety of common errors situations (including migrating from Swift 2
  // syntax).
  while (true) {

    if (Context.LangOpts.hasFeature(Feature::TrailingComma)) {
      // Condition terminator is `else` for `guard` statements.
      if (ParentKind == StmtKind::Guard && Tok.is(tok::kw_else)) {
        break;
      }
      // Condition terminator is start of statement body for `if` or `while`
      // statements. Missing `else` is a common mistake for `guard` statements
      // so we fall back to lookahead for a body.
      if (isStartOfConditionalStmtBody()) {
        break;
      }
    }

    Status |= parseStmtConditionElement(result, DefaultID, ParentKind,
                                        BindingKindStr);
    if (Status.isErrorOrHasCompletion())
      break;

    // If a comma exists consume it and succeed.
    if (consumeIf(tok::comma))
      continue;
    
    // If we have an "&&" token followed by a continuation of the statement
    // condition, then fixit the "&&" to "," and keep going.
    if (Tok.isAny(tok::oper_binary_spaced, tok::oper_binary_unspaced) &&
        Tok.getText() == "&&") {
      diagnose(Tok, diag::expected_comma_stmtcondition)
        .fixItReplaceChars(getEndOfPreviousLoc(), Tok.getRange().getEnd(), ",");
      consumeToken();
      continue;
    }

    // Boolean conditions are separated by commas, not the 'where' keyword, as
    // they were in Swift 2 and earlier.
    if (Tok.is(tok::kw_where)) {
      diagnose(Tok, diag::expected_comma_stmtcondition)
        .fixItReplaceChars(getEndOfPreviousLoc(), Tok.getRange().getEnd(), ",");
      consumeToken();
      continue;
    }
    
    break;
  }; 
  
  Condition = Context.AllocateCopy(result);
  return Status;
}

/// 
///   stmt-if:
///     'if' condition stmt-brace stmt-if-else?
///   stmt-if-else:
///    'else' stmt-brace
///    'else' stmt-if
ParserResult<Stmt> Parser::parseStmtIf(LabeledStmtInfo LabelInfo,
                                       bool IfWasImplicitlyInserted) {
  SourceLoc IfLoc;
  if (IfWasImplicitlyInserted) {
    // The code was invalid due to a missing 'if' (e.g. 'else x < y {') and a
    // fixit implicitly inserted it.
    IfLoc = Tok.getLoc();
  } else {
    IfLoc = consumeToken(tok::kw_if);
  }

  ParserStatus Status;
  StmtCondition Condition;
  ParserResult<BraceStmt> NormalBody;
  
  // A scope encloses the condition and true branch for any variables bound
  // by a conditional binding. The else branch does *not* see these variables.
  {
    auto recoverWithCond = [&](ParserStatus Status,
                               StmtCondition Condition) -> ParserResult<Stmt> {
      if (Condition.empty()) {
        SmallVector<StmtConditionElement, 1> ConditionElems;
        ConditionElems.emplace_back(new (Context) ErrorExpr(IfLoc));
        Condition = Context.AllocateCopy(ConditionElems);
      }
      auto EndLoc = Condition.back().getEndLoc();
      return makeParserResult(
          Status,
          new (Context) IfStmt(
              LabelInfo, IfLoc, Condition,
              BraceStmt::create(Context, EndLoc, {}, EndLoc, /*implicit=*/true),
              SourceLoc(), nullptr));
    };

    if (Tok.is(tok::l_brace)) {
      SourceLoc LBraceLoc = Tok.getLoc();
      diagnose(IfLoc, diag::missing_condition_after_if)
        .highlight(SourceRange(IfLoc, LBraceLoc));
      SmallVector<StmtConditionElement, 1> ConditionElems;
      ConditionElems.emplace_back(new (Context) ErrorExpr(LBraceLoc));
      Condition = Context.AllocateCopy(ConditionElems);
    } else {
      Status |= parseStmtCondition(Condition, diag::expected_condition_if,
                                   StmtKind::If);
      if (Status.isErrorOrHasCompletion())
        return recoverWithCond(Status, Condition);
    }

    if (Tok.is(tok::kw_else)) {
      SourceLoc ElseLoc = Tok.getLoc();
      diagnose(ElseLoc, diag::unexpected_else_after_if);
      diagnose(ElseLoc, diag::suggest_removing_else)
        .fixItRemove(ElseLoc);
      consumeToken(tok::kw_else);
    }

    NormalBody = parseBraceItemList(diag::expected_lbrace_after_if);
    Status |= NormalBody;
    if (NormalBody.isNull())
      return recoverWithCond(Status, Condition);
  }

  // The else branch, if any, is outside of the scope of the condition.
  SourceLoc ElseLoc;
  ParserResult<Stmt> ElseBody;
  if (Tok.is(tok::kw_else)) {
    ElseLoc = consumeToken(tok::kw_else);

    bool implicitlyInsertIf = false;
    if (Tok.isNot(tok::kw_if, tok::l_brace, tok::code_complete)) {
      // The code looks like 'if ... { ... } else not_if_or_lbrace', so we've
      // got a problem. If the last bit is 'else ... {' on one line, let's
      // assume they've forgotten the 'if'.
      BacktrackingScope backtrack(*this);
      if (skipUntilTokenOrEndOfLine(tok::l_brace, tok::r_brace))
        implicitlyInsertIf = Tok.is(tok::l_brace);
    }

    if (Tok.is(tok::kw_if) || implicitlyInsertIf) {
      if (implicitlyInsertIf) {
        diagnose(ElseLoc, diag::expected_lbrace_or_if_after_else_fixit)
            .fixItInsertAfter(ElseLoc, " if");
      }
      ElseBody = parseStmtIf(LabeledStmtInfo(), implicitlyInsertIf);
    } else if (Tok.is(tok::code_complete)) {
      if (CodeCompletionCallbacks) {
        CodeCompletionCallbacks->completeAfterIfStmtElse();
      }
      Status.setHasCodeCompletionAndIsError();
      consumeToken(tok::code_complete);
    } else {
      ElseBody = parseBraceItemList(diag::expected_lbrace_or_if_after_else);
    }
    Status |= ElseBody;
  }

  return makeParserResult(
      Status, new (Context) IfStmt(LabelInfo,
                                   IfLoc, Condition, NormalBody.get(),
                                   ElseLoc, ElseBody.getPtrOrNull()));
}

///   stmt-guard:
///     'guard' condition 'else' stmt-brace
///
ParserResult<Stmt> Parser::parseStmtGuard() {
  SourceLoc GuardLoc = consumeToken(tok::kw_guard);
  
  ParserStatus Status;
  StmtCondition Condition;
  ParserResult<BraceStmt> Body;

  auto recoverWithCond = [&](ParserStatus Status,
                             StmtCondition Condition) -> ParserResult<Stmt> {
    if (Condition.empty()) {
      SmallVector<StmtConditionElement, 1> ConditionElems;
      ConditionElems.emplace_back(new (Context) ErrorExpr(GuardLoc));
      Condition = Context.AllocateCopy(ConditionElems);
    }
    auto EndLoc = Condition.back().getEndLoc();
    return makeParserResult(
        Status,
        new (Context) GuardStmt(
            GuardLoc, Condition,
            BraceStmt::create(Context, EndLoc, {}, EndLoc, /*implicit=*/true)));
  };

  if (Tok.isAny(tok::l_brace, tok::kw_else)) {
    SourceLoc LBraceLoc = Tok.getLoc();
    diagnose(GuardLoc, diag::missing_condition_after_guard)
      .highlight(SourceRange(GuardLoc, LBraceLoc));
    SmallVector<StmtConditionElement, 1> ConditionElems;
    ConditionElems.emplace_back(new (Context) ErrorExpr(LBraceLoc));
    Condition = Context.AllocateCopy(ConditionElems);
  } else {
    Status |= parseStmtCondition(Condition, diag::expected_condition_guard,
                                 StmtKind::Guard);
    if (Status.isErrorOrHasCompletion()) {
      // FIXME: better recovery
      return recoverWithCond(Status, Condition);
    }
  }

  // Parse the 'else'.  If it is missing, and if the following token isn't a {
  // then the parser is hopelessly lost - just give up instead of spewing.
  if (!consumeIf(tok::kw_else)) {
    checkForInputIncomplete();
    auto diag = diagnose(Tok, diag::expected_else_after_guard);
    if (Tok.is(tok::l_brace))
      diag.fixItInsert(Tok.getLoc(), "else ");
    else
      return recoverWithCond(Status, Condition);
  }

  Body = parseBraceItemList(diag::expected_lbrace_after_guard);
  if (Body.isNull())
    return recoverWithCond(Status, Condition);

  Status |= Body;
  
  return makeParserResult(Status,
              new (Context) GuardStmt(GuardLoc, Condition, Body.get()));
}

/// 
///   stmt-while:
///     (identifier ':')? 'while' expr-basic stmt-brace
ParserResult<Stmt> Parser::parseStmtWhile(LabeledStmtInfo LabelInfo) {
  SourceLoc WhileLoc = consumeToken(tok::kw_while);

  ParserStatus Status;
  StmtCondition Condition;

  auto recoverWithCond = [&](ParserStatus Status,
                             StmtCondition Condition) -> ParserResult<Stmt> {
    if (Condition.empty()) {
      SmallVector<StmtConditionElement, 1> ConditionElems;
      ConditionElems.emplace_back(new (Context) ErrorExpr(WhileLoc));
      Condition = Context.AllocateCopy(ConditionElems);
    }
    auto EndLoc = Condition.back().getEndLoc();
    return makeParserResult(
        Status,
        new (Context) WhileStmt(
            LabelInfo, WhileLoc, Condition,
            BraceStmt::create(Context, EndLoc, {}, EndLoc, /*implicit=*/true)));
  };

  if (Tok.is(tok::l_brace)) {
    SourceLoc LBraceLoc = Tok.getLoc();
    diagnose(WhileLoc, diag::missing_condition_after_while)
      .highlight(SourceRange(WhileLoc, LBraceLoc));
    SmallVector<StmtConditionElement, 1> ConditionElems;
    ConditionElems.emplace_back(new (Context) ErrorExpr(LBraceLoc));
    Condition = Context.AllocateCopy(ConditionElems);
  } else {
    Status |= parseStmtCondition(Condition, diag::expected_condition_while,
                                 StmtKind::While);
    if (Status.isErrorOrHasCompletion())
      return recoverWithCond(Status, Condition);
  }

  ParserResult<BraceStmt> Body =
      parseBraceItemList(diag::expected_lbrace_after_while);
  Status |= Body;
  if (Body.isNull())
    return recoverWithCond(Status, Condition);

  return makeParserResult(
      Status, new (Context) WhileStmt(LabelInfo, WhileLoc, Condition,
                                      Body.get()));
}

///
///   stmt-repeat:
///     (identifier ':')? 'repeat' stmt-brace 'while' expr
ParserResult<Stmt> Parser::parseStmtRepeat(LabeledStmtInfo labelInfo) {
  SourceLoc repeatLoc = consumeToken(tok::kw_repeat);

  ParserStatus status;

  ParserResult<BraceStmt> body =
      parseBraceItemList(diag::expected_lbrace_after_repeat);
  status |= body;
  if (body.isNull())
    body = makeParserResult(
        body, BraceStmt::create(Context, repeatLoc, {}, PreviousLoc, true));

  SourceLoc whileLoc;

  if (!consumeIf(tok::kw_while, whileLoc)) {
    diagnose(body.getPtrOrNull()->getEndLoc(),
             diag::expected_while_after_repeat_body);
    return body;
  }

  ParserResult<Expr> condition;
  if (Tok.is(tok::l_brace)) {
    diagnose(whileLoc, diag::missing_condition_after_while);
    condition = makeParserErrorResult(new (Context) ErrorExpr(whileLoc));
  } else {
    condition = parseExpr(diag::expected_expr_repeat_while);
    status |= condition;
    if (condition.isNull()) {
      condition = makeParserErrorResult(new (Context) ErrorExpr(whileLoc));
    }
  }

  return makeParserResult(
      status,
      new (Context) RepeatWhileStmt(labelInfo, repeatLoc, condition.get(),
                                    whileLoc, body.get()));
}

/// 
///   stmt-do:
///     (identifier ':')? 'do' throws-clause? stmt-brace
///     (identifier ':')? 'do' throws-clause? stmt-brace stmt-catch+
ParserResult<Stmt> Parser::parseStmtDo(LabeledStmtInfo labelInfo,
                                       bool shouldSkipDoTokenConsume) {
  SourceLoc doLoc;

  if (shouldSkipDoTokenConsume) {
    doLoc = Tok.getLoc();
  } else {
    doLoc = consumeToken(tok::kw_do);
  }

  ParserStatus status;

  // Parse the optional 'throws' clause.
  SourceLoc throwsLoc;
  TypeRepr *thrownType = nullptr;
  if (consumeIf(tok::kw_throws, throwsLoc)) {
    // Parse the thrown error type.
    SourceLoc lParenLoc;
    if (consumeIf(tok::l_paren, lParenLoc)) {
      ParserResult<TypeRepr> parsedThrownTy =
          parseType(diag::expected_thrown_error_type);
      thrownType = parsedThrownTy.getPtrOrNull();
      status |= parsedThrownTy;

      SourceLoc rParenLoc;
      parseMatchingToken(
          tok::r_paren, rParenLoc,
          diag::expected_rparen_after_thrown_error_type, lParenLoc);
    }
  }

  ParserResult<BraceStmt> body =
      parseBraceItemList(diag::expected_lbrace_after_do);
  status |= body;
  if (body.isNull())
    body = makeParserResult(
        body, BraceStmt::create(Context, doLoc, {}, PreviousLoc, true));

  // If the next token is 'catch', this is a 'do'/'catch' statement.
  if (Tok.is(tok::kw_catch)) {
    // Parse 'catch' clauses
    SmallVector<CaseStmt *, 4> allClauses;
    ParserStatus catchClausesStatus;
    do {
      ParserResult<CaseStmt> clause = parseStmtCatch();
      catchClausesStatus |= clause;
      if (catchClausesStatus.hasCodeCompletion() && clause.isNull()) {
        status |= catchClausesStatus;
        return makeParserResult<Stmt>(status, nullptr);
      }

      // parseStmtCatch promises to return non-null unless we are
      // completing inside the catch's pattern.
      allClauses.push_back(clause.get());
    } while (Tok.is(tok::kw_catch) && !catchClausesStatus.hasCodeCompletion());
    status |= catchClausesStatus;

    // Recover from all of the clauses failing to parse by returning a
    // normal do-statement.
    if (allClauses.empty()) {
      assert(status.isErrorOrHasCompletion());
      return makeParserResult(status,
                        new (Context) DoStmt(labelInfo, doLoc, body.get()));
    }

    return makeParserResult(status,
      DoCatchStmt::create(CurDeclContext, labelInfo, doLoc, throwsLoc,
                          thrownType, body.get(), allClauses));
  }

  if (throwsLoc.isValid()) {
    diagnose(throwsLoc, diag::do_throws_without_catch);
  }

  // If we dont see a 'while' or see a 'while' that starts
  // from new line. This is just the bare `do` scoping statement.
  if (Tok.getKind() != tok::kw_while || Tok.isAtStartOfLine()) {
    return makeParserResult(status,
                            new (Context) DoStmt(labelInfo, doLoc, body.get()));
  }
  SourceLoc whileLoc = Tok.getLoc();
  // But if we do, advise the programmer that it's 'repeat' now.
  diagnose(doLoc, diag::do_while_now_repeat_while);
  diagnose(doLoc, diag::do_while_expected_repeat_while)
    .fixItReplace(doLoc, "repeat");
  diagnose(doLoc, diag::do_while_expected_separate_stmt)
    .fixItInsert(whileLoc, "\n");

  consumeToken(tok::kw_while);
  status.setIsParseError();
  ParserResult<Expr> condition;
  if (Tok.is(tok::l_brace)) {
    SourceLoc lbraceLoc = Tok.getLoc();
    diagnose(whileLoc, diag::missing_condition_after_while);
    condition = makeParserErrorResult(new (Context) ErrorExpr(lbraceLoc));
  } else {
    condition = parseExpr(diag::expected_expr_repeat_while);
    status |= condition;
    if (condition.isNull() || condition.hasCodeCompletion())
      return makeParserResult<Stmt>(status, nullptr); // FIXME: better recovery
  }

  return makeParserResult(
      status,
      new (Context) RepeatWhileStmt(labelInfo, doLoc, condition.get(), whileLoc,
                                body.get()));
}

///  stmt-catch:
///    'catch' pattern ('where' expr)? stmt-brace
///
/// Note that this is not a "first class" statement; it can only
/// appear following a 'do' statement.
///
/// This routine promises to return a non-null result unless there was
/// a code-completion token in the pattern.
ParserResult<CaseStmt> Parser::parseStmtCatch() {
  // A catch block has its own scope for variables bound out of the pattern.
  SourceLoc catchLoc = consumeToken(tok::kw_catch);

  SmallVector<VarDecl*, 4> boundDecls;
  ParserStatus status;
  SmallVector<CaseLabelItem, 1> caseLabelItems;

  {
    bool isFirst = true;
    while (true) {
      GuardedPattern PatternResult;
      parseGuardedPattern(*this, PatternResult, status, boundDecls,
                          GuardedPatternContext::Catch, isFirst);
      caseLabelItems.emplace_back(PatternResult.ThePattern,
                                  PatternResult.WhereLoc, PatternResult.Guard);
      isFirst = false;
      if (!consumeIf(tok::comma))
        break;
    }
  }

  auto bodyResult = parseBraceItemList(diag::expected_lbrace_after_catch);
  status |= bodyResult;
  if (bodyResult.isNull()) {
    bodyResult = makeParserErrorResult(BraceStmt::create(Context, PreviousLoc,
                                                         {}, PreviousLoc,
                                                         /*implicit=*/ true));
  }

  return makeParserResult(
      status, CaseStmt::createParsedDoCatch(Context, catchLoc, caseLabelItems,
                                            bodyResult.get()));
}

static bool isStmtForCStyle(Parser &P) {
  // If we have a leading identifier followed by a ':' or 'in', or have a
  // 'case', then this is obviously a for-each loop. "for in ..." is malformed
  // but it's obviously not a C-style for.
  if ((P.Tok.isIdentifierOrUnderscore() &&
         P.peekToken().isAny(tok::colon, tok::kw_in)) ||
      P.Tok.isAny(tok::kw_case, tok::kw_in))
    return false;

  // Otherwise, we have to look forward if we see ';' in control part.
  Parser::BacktrackingScope Backtrack(P);

  // The condition of a c-style-for loop can be parenthesized.
  auto HasLParen = P.consumeIf(tok::l_paren);

  // Skip until we see ';', or something that ends control part.
  while (true) {
    if (P.Tok.isAny(tok::eof, tok::kw_in, tok::l_brace, tok::r_brace,
                    tok::r_paren) ||
        P.isStartOfStmt(/*preferExpr*/ false))
      return false;
    // If we saw newline before ';', consider it is a foreach statement.
    if (!HasLParen && P.Tok.isAtStartOfLine())
      return false;
    if (P.Tok.is(tok::semi))
      return true;
    P.skipSingle();
  }
}

/// 
///   stmt-for-each:
///     (identifier ':')? 'for' pattern 'in' expr-basic \
///             ('where' expr-basic)? stmt-brace
ParserResult<Stmt> Parser::parseStmtForEach(LabeledStmtInfo LabelInfo) {
  SourceLoc ForLoc = consumeToken(tok::kw_for);
  ParserStatus Status;
  ParserResult<Pattern> pattern;
  ParserResult<Expr> Container;

  // The C-style for loop which was supported in Swift2 and foreach-style-for
  // loop are conflated together into a single keyword, so we have to do some
  // lookahead to resolve what is going on.
  bool IsCStyleFor = isStmtForCStyle(*this);
  auto StartOfControl = Tok.getLoc();
  SourceLoc AwaitLoc;
  SourceLoc TryLoc;

  if (Tok.isContextualKeyword("await")) {
    AwaitLoc = consumeToken();
  } else if (Tok.is(tok::kw_try)) {
    TryLoc = consumeToken();
    if (Tok.isContextualKeyword("await")) {
      AwaitLoc = consumeToken();
    }
  }

  if (Tok.is(tok::code_complete)) {
    if (CodeCompletionCallbacks) {
      CodeCompletionCallbacks->completeForEachPatternBeginning(
          TryLoc.isValid(), AwaitLoc.isValid());
    }
    consumeToken(tok::code_complete);
    // Since 'completeForeachPatternBeginning' is a keyword only completion,
    // we don't need to parse the rest of 'for' statement.
    return makeParserCodeCompletionStatus();
  }

  // Parse the pattern.  This is either 'case <refutable pattern>' or just a
  // normal pattern.
  if (consumeIf(tok::kw_case)) {
    llvm::SaveAndRestore<decltype(InBindingPattern)> T(
        InBindingPattern, PatternBindingState::InMatchingPattern);

    // Reset async attribute in parser context.
    llvm::SaveAndRestore<bool> AsyncAttr(InPatternWithAsyncAttribute, false);

    pattern = parseMatchingPattern(/*isExprBasic*/true);
    pattern = parseOptionalPatternTypeAnnotation(pattern);
  } else if (!IsCStyleFor || Tok.is(tok::kw_var)) {
    // Change the parser state to know that the pattern we're about to parse is
    // implicitly mutable.  Bound variables can be changed to mutable explicitly
    // if desired by using a 'var' pattern.
    assert(InBindingPattern == PatternBindingState::NotInBinding &&
           "for-each loops cannot exist inside other patterns");
    InBindingPattern = PatternBindingState::ImplicitlyImmutable;
    pattern = parseTypedPattern();
    assert(InBindingPattern == PatternBindingState::ImplicitlyImmutable);
    InBindingPattern = PatternBindingState::NotInBinding;
  }
  
  SourceLoc InLoc;
  if (pattern.isNull()) {
    // Recover by creating a "_" pattern.
    pattern = makeParserErrorResult(AnyPattern::createImplicit(Context));
    consumeIf(tok::kw_in, InLoc);
  } else if (!IsCStyleFor) {
    parseToken(tok::kw_in, InLoc, diag::expected_foreach_in);
  }

  if (IsCStyleFor) {
    // Skip until start of body part.
    if (Tok.is(tok::l_paren)) {
      skipSingle();
    } else {
      // If not parenthesized, don't run over the line.
      while (Tok.isNot(tok::eof, tok::r_brace, tok::l_brace, tok::code_complete)
             && !Tok.isAtStartOfLine())
        skipSingle();
    }
    if (Tok.is(tok::code_complete))
      return makeParserCodeCompletionStatus();

    assert(StartOfControl != Tok.getLoc());
    SourceRange ControlRange(StartOfControl, PreviousLoc);
    Container = makeParserErrorResult(new (Context) ErrorExpr(ControlRange));
    diagnose(ForLoc, diag::c_style_for_stmt_removed)
      .highlight(ControlRange);
    Status = makeParserError();
  } else if (Tok.is(tok::l_brace)) {
    SourceLoc LBraceLoc = Tok.getLoc();
    diagnose(LBraceLoc, diag::expected_foreach_container);
    Container = makeParserErrorResult(new (Context) ErrorExpr(LBraceLoc));
  } else if (Tok.is(tok::code_complete)) {
    // If there is no "in" keyword, suggest it. Otherwise, complete the
    // sequence.
    if (InLoc.isInvalid()) {
      if (CodeCompletionCallbacks) {
        CodeCompletionCallbacks->completeForEachInKeyword();
      }
      consumeToken(tok::code_complete);
      return makeParserCodeCompletionStatus();
    } else {
      Container =
          makeParserResult(new (Context) CodeCompletionExpr(Tok.getLoc()));
      Container.setHasCodeCompletionAndIsError();
      Status |= Container;
      if (CodeCompletionCallbacks) {
        CodeCompletionCallbacks->completeForEachSequenceBeginning(
            cast<CodeCompletionExpr>(Container.get()));
      }
      consumeToken(tok::code_complete);
    }
  } else {
    Container = parseExprBasic(diag::expected_foreach_container);
    Status |= Container;
    if (Container.isNull())
      Container = makeParserErrorResult(new (Context) ErrorExpr(Tok.getLoc()));
    if (Container.isParseErrorOrHasCompletion())
      // Recover.
      skipUntilDeclStmtRBrace(tok::l_brace, tok::kw_where);
  }
  
  // Parse the 'where' expression if present.
  ParserResult<Expr> Where;
  SourceLoc WhereLoc;
  if (Tok.is(tok::kw_where)) {
    WhereLoc = consumeToken();
    Where = parseExprBasic(diag::expected_foreach_where_expr);
    if (Where.isNull())
      Where = makeParserErrorResult(new (Context) ErrorExpr(Tok.getLoc()));
    Status |= Where;
  }

  // stmt-brace
  ParserResult<BraceStmt> Body =
      parseBraceItemList(diag::expected_foreach_lbrace);
  Status |= Body;
  if (Body.isNull())
    Body = makeParserResult(
        Body, BraceStmt::create(Context, ForLoc, {}, PreviousLoc, true));

  return makeParserResult(
      Status,
      new (Context) ForEachStmt(LabelInfo, ForLoc, TryLoc, AwaitLoc, pattern.get(), InLoc,
                                Container.get(), WhereLoc, Where.getPtrOrNull(),
                                Body.get()));
}

///
///    stmt-switch:
///      (identifier ':')? 'switch' expr-basic '{' stmt-case+ '}'
ParserResult<Stmt> Parser::parseStmtSwitch(LabeledStmtInfo LabelInfo) {
  SourceLoc SwitchLoc = consumeToken(tok::kw_switch);

  ParserStatus Status;
  ParserResult<Expr> SubjectExpr;
  SourceLoc SubjectLoc = Tok.getLoc();
  if (Tok.is(tok::l_brace)) {
    diagnose(SubjectLoc, diag::expected_switch_expr);
    SubjectExpr = makeParserErrorResult(new (Context) ErrorExpr(SubjectLoc));
  } else {
    SubjectExpr = parseExprBasic(diag::expected_switch_expr);
    if (SubjectExpr.isNull()) {
      SubjectExpr = makeParserErrorResult(new (Context) ErrorExpr(SubjectLoc));
    }
    Status |= SubjectExpr;
  }

  SourceLoc lBraceLoc;
  SourceLoc rBraceLoc;
  SmallVector<ASTNode, 8> cases;

  if (Status.isErrorOrHasCompletion()) {
    return makeParserResult(
        Status, SwitchStmt::create(LabelInfo, SwitchLoc, SubjectExpr.get(),
                                   lBraceLoc, cases, rBraceLoc,
                                   /*EndLoc=*/PreviousLoc, Context));
  }

  if (!consumeIf(tok::l_brace, lBraceLoc)) {
    diagnose(Tok, diag::expected_lbrace_after_switch);
    return makeParserResult(
        Status, SwitchStmt::create(LabelInfo, SwitchLoc, SubjectExpr.get(),
                                   lBraceLoc, cases, rBraceLoc,
                                   /*EndLoc=*/PreviousLoc, Context));
  }

  Status |= parseStmtCases(cases, /*IsActive=*/true);

  // We cannot have additional cases after a default clause. Complain on
  // the first offender.
  bool hasDefault = false;
  for (auto Element : cases) {
    if (!Element.is<Stmt*>()) continue;
    auto *CS = cast<CaseStmt>(Element.get<Stmt*>());
    if (hasDefault) {
      diagnose(CS->getLoc(), diag::case_after_default);
      break;
    }
    hasDefault |= CS->isDefault();
  }

  if (parseMatchingToken(tok::r_brace, rBraceLoc,
                         diag::expected_rbrace_switch, lBraceLoc)) {
    Status.setIsParseError();
  }

  return makeParserResult(
      Status, SwitchStmt::create(LabelInfo, SwitchLoc, SubjectExpr.get(),
                                 lBraceLoc, cases, rBraceLoc,
                                 /*EndLoc=*/rBraceLoc, Context));
}

ParserStatus
Parser::parseStmtCases(SmallVectorImpl<ASTNode> &cases, bool IsActive) {
  ParserStatus Status;
  while (Tok.isNot(tok::r_brace, tok::eof,
                   tok::pound_endif, tok::pound_elseif, tok::pound_else)) {
    if (isAtStartOfSwitchCase(*this)) {
      ParserResult<CaseStmt> Case = parseStmtCase(IsActive);
      Status |= Case;
      if (Case.isNonNull())
        cases.emplace_back(Case.get());
    } else if (Tok.is(tok::pound_if)) {
      // '#if' in 'case' position can enclose one or more 'case' or 'default'
      // clauses.
      auto IfConfigResult =
          parseIfConfig(IfConfigContext::SwitchStmt,
                        [&](bool IsActive) {
                          SmallVector<ASTNode, 16> elements;
                          parseStmtCases(elements, IsActive);

                          if (IsActive) {
                            cases.append(elements);
                          }
                        });
      Status |= IfConfigResult;
    } else if (Tok.is(tok::pound_warning) || Tok.is(tok::pound_error)) {
      auto PoundDiagnosticResult = parseDeclPoundDiagnostic();
      Status |= PoundDiagnosticResult;
      if (auto PDD = PoundDiagnosticResult.getPtrOrNull()) {
        cases.emplace_back(PDD);
      }
    } else if (Tok.is(tok::code_complete)) {
      if (CodeCompletionCallbacks) {
        CodeCompletionCallbacks->completeCaseStmtKeyword();
      }
      consumeToken(tok::code_complete);
      return makeParserCodeCompletionStatus();
    } else {
      // If there are non-case-label statements at the start of the switch body,
      // raise an error and recover by discarding them.
      diagnose(Tok, diag::stmt_in_switch_not_covered_by_case);

      while (Tok.isNot(tok::r_brace, tok::eof, tok::pound_elseif,
                       tok::pound_else, tok::pound_endif) &&
             !isTerminatorForBraceItemListKind(BraceItemListKind::Case, {})) {
        skipSingle();
      }
    }
  }
  return Status;
}

static ParserStatus parseStmtCase(Parser &P, SourceLoc &CaseLoc,
                                  SmallVectorImpl<CaseLabelItem> &LabelItems,
                                  SmallVectorImpl<VarDecl *> &BoundDecls,
                                  SourceLoc &ColonLoc) {
  ParserStatus Status;
  CaseLoc = P.consumeToken(tok::kw_case);

  {
    bool isFirst = true;
    while (true) {
      GuardedPattern PatternResult;
      parseGuardedPattern(P, PatternResult, Status, BoundDecls,
                          GuardedPatternContext::Case, isFirst);
      LabelItems.emplace_back(PatternResult.ThePattern, PatternResult.WhereLoc,
                              PatternResult.Guard);
      isFirst = false;
      if (!P.consumeIf(tok::comma))
        break;

      if (P.Context.LangOpts.hasFeature(Feature::TrailingComma) &&
          P.Tok.is(tok::colon)) {
        break;
      }

      if (P.Tok.is(tok::kw_case)) {
        P.diagnose(P.Tok, diag::extra_case_keyword)
          .fixItRemove(SourceRange(P.Tok.getLoc()));
        P.consumeToken(tok::kw_case);
      }
    }
  }

  ColonLoc = P.Tok.getLoc();
  if (!P.Tok.is(tok::colon)) {
    P.diagnose(P.Tok, diag::expected_case_colon, "case");
    Status.setIsParseError();
  } else
    P.consumeToken(tok::colon);

  return Status;
}

static ParserStatus
parseStmtCaseDefault(Parser &P, SourceLoc &CaseLoc,
                     SmallVectorImpl<CaseLabelItem> &LabelItems,
                     SourceLoc &ColonLoc) {
  ParserStatus Status;

  CaseLoc = P.consumeToken(tok::kw_default);

  // We don't allow 'where' guards on a 'default' block. For recovery
  // parse one if present.
  SourceLoc WhereLoc;
  ParserResult<Expr> Guard;
  if (P.Tok.is(tok::kw_where)) {
    P.diagnose(P.Tok, diag::default_with_where);
    WhereLoc = P.consumeToken(tok::kw_where);
    Guard = P.parseExpr(diag::expected_case_where_expr);
    Status |= Guard;
  }

  ColonLoc = P.Tok.getLoc();
  if (!P.Tok.is(tok::colon)) {
    P.diagnose(P.Tok, diag::expected_case_colon, "default");
    Status.setIsParseError();
  } else
    P.consumeToken(tok::colon);

  // Create an implicit AnyPattern to represent the default match.
  auto Any = new (P.Context) AnyPattern(CaseLoc);
  if (CaseLoc.isInvalid())
    Any->setImplicit();
  LabelItems.push_back(
      CaseLabelItem::getDefault(Any, WhereLoc, Guard.getPtrOrNull()));

  return Status;
}

ParserResult<CaseStmt> Parser::parseStmtCase(bool IsActive) {
  ParserStatus Status;

  SmallVector<CaseLabelItem, 2> CaseLabelItems;
  SmallVector<VarDecl *, 4> BoundDecls;

  SourceLoc UnknownAttrLoc;
  while (Tok.is(tok::at_sign)) {
    if (peekToken().isContextualKeyword("unknown")) {
      if (!UnknownAttrLoc.isValid()) {
        UnknownAttrLoc = consumeToken(tok::at_sign);
      } else {
        diagnose(Tok, diag::duplicate_attribute, false);
        diagnose(UnknownAttrLoc, diag::previous_attribute, false);
        consumeToken(tok::at_sign);
      }
      consumeToken(tok::identifier);

      if (Tok.is(tok::l_paren)) {
        diagnose(Tok, diag::unexpected_lparen_in_attribute, "unknown");
        skipSingle();
      }
    } else {
      assert(peekToken().is(tok::identifier) && "isAtStartOfSwitchCase() lied");

      consumeToken(tok::at_sign);
      diagnose(Tok, diag::unknown_attribute, Tok.getText());
      consumeToken(tok::identifier);

      if (Tok.is(tok::l_paren))
        skipSingle();
    }
  }

  SourceLoc CaseLoc;
  SourceLoc ColonLoc;
  if (Tok.is(tok::kw_case)) {
    Status |=
        ::parseStmtCase(*this, CaseLoc, CaseLabelItems, BoundDecls, ColonLoc);
  } else if (Tok.is(tok::kw_default)) {
    Status |= parseStmtCaseDefault(*this, CaseLoc, CaseLabelItems, ColonLoc);
  } else {
    llvm_unreachable("isAtStartOfSwitchCase() lied.");
  }

  assert(!CaseLabelItems.empty() && "did not parse any labels?!");

  SmallVector<ASTNode, 8> BodyItems;

  SourceLoc StartOfBody = Tok.getLoc();
  if (Tok.isNot(tok::r_brace) && !isAtStartOfSwitchCase(*this)) {
    Status |= parseBraceItems(BodyItems, BraceItemListKind::Case);
  } else if (Status.isSuccess() && !Status.hasCodeCompletion()) {
    diagnose(CaseLoc, diag::case_stmt_without_body,
             CaseLabelItems.back().isDefault())
        .highlight(SourceRange(CaseLoc, ColonLoc))
        .fixItInsertAfter(ColonLoc, " break");
  }
  BraceStmt *Body;
  if (BodyItems.empty()) {
    Body = BraceStmt::create(Context, PreviousLoc, ArrayRef<ASTNode>(),
                             PreviousLoc, /*implicit=*/true);
  } else {
    Body = BraceStmt::create(Context, StartOfBody, BodyItems,
                             PreviousLoc, /*implicit=*/true);
  }

  return makeParserResult(
      Status, CaseStmt::createParsedSwitchCase(Context, CaseLoc, CaseLabelItems,
                                               UnknownAttrLoc, ColonLoc, Body));
}

/// stmt-pound-assert:
///   '#assert' '(' expr (',' string_literal)? ')'
ParserResult<Stmt> Parser::parseStmtPoundAssert() {
  SourceLoc startLoc = consumeToken(tok::pound_assert);
  SourceLoc endLoc;

  if (Tok.isNot(tok::l_paren)) {
    diagnose(Tok, diag::pound_assert_expected_lparen);
    return makeParserError();
  }
  SourceLoc LBLoc = consumeToken(tok::l_paren);

  auto conditionExprResult = parseExpr(diag::pound_assert_expected_expression);
  if (conditionExprResult.isParseErrorOrHasCompletion())
    return ParserStatus(conditionExprResult);

  StringRef message;
  if (consumeIf(tok::comma)) {
    if (Tok.isNot(tok::string_literal)) {
      diagnose(Tok.getLoc(), diag::pound_assert_expected_string_literal);
      return makeParserError();
    }

    auto messageOpt = getStringLiteralIfNotInterpolated(Tok.getLoc(),
                                                        "'#assert' message");
    consumeToken();
    if (!messageOpt)
      return makeParserError();

    message = *messageOpt;
  }

  if (parseMatchingToken(tok::r_paren, endLoc,
                         diag::pound_assert_expected_rparen, LBLoc)) {
    return makeParserError();
  }

  if (!Context.LangOpts.hasFeature(Feature::StaticAssert)) {
    diagnose(startLoc, diag::pound_assert_disabled);
    return makeParserError();
  }

  return makeParserResult<Stmt>(new (Context) PoundAssertStmt(
      SourceRange(startLoc, endLoc), conditionExprResult.get(), message));
}