ASTWalker.cpp

//===--- ASTWalker.cpp - AST Traversal ------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
//  This file implements a recursive traversal of every node in an AST.
//
//  It's important to update this traversal whenever the AST is
//  changed, whether by adding a new node class or adding a new child
//  to an existing node.  Many walker implementations rely on being
//  invoked with every node in the AST.
//
//  Please follow these general rules when implementing traversal for
//  a node:
//
//    - Every node should be walked.  If a node has both syntactic and
//      semantic components, you should make sure you visit every node
//      in both.
//
//    - Nodes should only be walked once.  So if a node has both
//      syntactic and semantic components, but the type-checker builds
//      the semantic components directly on top of the syntactic
//      components, walking the semantic components will be sufficient
//      to visit all the nodes in both.
//
//    - Explicitly-written nodes should be walked in left-to-right
//      syntactic order.  The ordering of implicit nodes isn't
//      particularly important.
//
//      Note that semantic components will generally preserve the
//      syntactic order of their children because doing something else
//      could illegally change order of evaluation.  This is why, for
//      example, shuffling a TupleExpr creates a DestructureTupleExpr
//      instead of just making a new TupleExpr with the elements in
//      different order.
//
//    - Sub-expressions and sub-statements should be replaceable.
//      It's reasonable to expect that the replacement won't be
//      completely unrelated to the original, but try to avoid making
//      assumptions about the exact representation type.  For example,
//      assuming that a child expression is literally a TupleExpr may
//      only be a reasonable assumption in an unchecked parse tree.
//
//    - Avoid relying on the AST being type-checked or even
//      well-formed during traversal.
//
//===----------------------------------------------------------------------===//

#include "swift/AST/ASTWalker.h"
#include "swift/AST/ASTVisitor.h"
#include "swift/AST/GenericParamList.h"
#include "swift/AST/ParameterList.h"
#include "swift/AST/PrettyStackTrace.h"
using namespace swift;

void ASTWalker::anchor() {}

namespace {

/// Traversal - This class implements a simple expression/statement
/// recursive traverser which queries a user-provided walker class
/// on every node in an AST.
class Traversal : public ASTVisitor<Traversal, Expr*, Stmt*,
                                    /*Decl*/ bool,
                                    Pattern *, /*TypeRepr*/ bool>
{
  friend class ASTVisitor<Traversal, Expr*, Stmt*, bool, Pattern*, bool>;
  typedef ASTVisitor<Traversal, Expr*, Stmt*, bool, Pattern*, bool> inherited;

  ASTWalker &Walker;
  
  /// RAII object that sets the parent of the walk context 
  /// appropriately.
  class SetParentRAII {
    ASTWalker &Walker;
    decltype(ASTWalker::Parent) PriorParent;
    
  public:
    template<typename T>
    SetParentRAII(ASTWalker &walker, T *newParent)
      : Walker(walker), PriorParent(walker.Parent) {
      walker.Parent = newParent;
    }
    
    ~SetParentRAII() {
      Walker.Parent = PriorParent;
    }
  };

  [[nodiscard]]
  Expr *visit(Expr *E) {
    SetParentRAII SetParent(Walker, E);
    return inherited::visit(E);
  }

  [[nodiscard]]
  Stmt *visit(Stmt *S) {
    SetParentRAII SetParent(Walker, S);
    return inherited::visit(S);
  }

  [[nodiscard]]
  Pattern *visit(Pattern *P) {
    SetParentRAII SetParent(Walker, P);
    return inherited::visit(P);
  }

  [[nodiscard]]
  bool visit(Decl *D) {
    SetParentRAII SetParent(Walker, D);
    return inherited::visit(D);
  }

  [[nodiscard]]
  bool visit(TypeRepr *T) {
    SetParentRAII SetParent(Walker, T);
    return inherited::visit(T);
  }

  [[nodiscard]]
  bool visit(ParameterList *PL) {
    return inherited::visit(PL);
  }

  //===--------------------------------------------------------------------===//
  //                                 Decls
  //===--------------------------------------------------------------------===//

  [[nodiscard]]
  bool visitGenericParamListIfNeeded(GenericContext *GC) {
    // Must check this first in case extensions have not been bound yet
    if (Walker.shouldWalkIntoGenericParams()) {
      if (auto *params = GC->getParsedGenericParams()) {
        if (doIt(params))
          return true;
      }
      return true;
    }
    return false;
  }

  [[nodiscard]]
  bool visitTrailingRequirements(GenericContext *GC) {
    if (const auto Where = GC->getTrailingWhereClause()) {
      for (auto &Req: Where->getRequirements())
        if (doIt(Req))
          return true;
    }
    return false;
  }

  bool visitImportDecl(ImportDecl *ID) {
    return false;
  }

  bool visitExtensionDecl(ExtensionDecl *ED) {
    if (auto *typeRepr = ED->getExtendedTypeRepr())
      if (doIt(typeRepr))
        return true;
    auto inheritedTypes = ED->getInherited();
    for (auto i : inheritedTypes.getIndices()) {
      if (auto *const TyR = inheritedTypes.getTypeRepr(i))
        if (doIt(TyR))
          return true;
    }
    if (visitTrailingRequirements(ED))
      return true;

    for (Decl *M : ED->getMembers()) {
      if (doIt(M))
        return true;

      if (Walker.shouldWalkAccessorsTheOldWay()) {
        // Pretend that accessors share a parent with the storage.
        //
        // FIXME: Update existing ASTWalkers to deal with accessors appearing as
        // children of the storage instead.
        if (auto *ASD = dyn_cast<AbstractStorageDecl>(M)) {
          for (auto AD : ASD->getAllAccessors()) {
            if (doIt(AD))
              return true;
          }
        }
      }
    }
    return false;
  }

  bool visitPatternBindingDecl(PatternBindingDecl *PBD) {
    bool isPropertyWrapperBackingProperty = false;
    if (auto singleVar = PBD->getSingleVar()) {
      isPropertyWrapperBackingProperty =
        singleVar->getOriginalWrappedProperty() != nullptr;
    }

    for (auto idx : range(PBD->getNumPatternEntries())) {
      if (Pattern *Pat = doIt(PBD->getPattern(idx)))
        PBD->setPattern(idx, Pat, PBD->getInitContext(idx));
      else
        return true;

      if (!PBD->getInit(idx) || isPropertyWrapperBackingProperty)
        continue;

      if (PBD->isInitializerSubsumed(idx) &&
          Walker.getLazyInitializerWalkingBehavior() !=
              LazyInitializerWalking::InPatternBinding) {
        break;
      }

#ifndef NDEBUG
      PrettyStackTraceDecl debugStack("walking into initializer for", PBD);
#endif
      if (Expr *E2 = doIt(PBD->getInit(idx)))
        PBD->setInit(idx, E2);
      else
        return true;
    }
    return false;
  }

  bool visitEnumCaseDecl(EnumCaseDecl *ECD) {
    // We'll visit the EnumElementDecls separately.
    return false;
  }

  bool visitTopLevelCodeDecl(TopLevelCodeDecl *TLCD) {
    if (BraceStmt *S = cast_or_null<BraceStmt>(doIt(TLCD->getBody()))) {
      TLCD->setBody(S);
      return false;
    }
    return true;
  }

  bool visitIfConfigDecl(IfConfigDecl *ICD) {
    // By default, just visit the elements that are actually
    // injected into the enclosing context.
    return false;
  }

  bool visitPoundDiagnosticDecl(PoundDiagnosticDecl *PDD) {
    // By default, ignore #error/#warning.
    return false;
  }

  bool visitOperatorDecl(OperatorDecl *OD) {
    return false;
  }

  bool visitPrecedenceGroupDecl(PrecedenceGroupDecl *PGD) {
    return false;
  }

  bool visitTypeAliasDecl(TypeAliasDecl *TAD) {
    bool WalkGenerics = visitGenericParamListIfNeeded(TAD);

    if (auto typerepr = TAD->getUnderlyingTypeRepr())
      if (doIt(typerepr))
        return true;

    return WalkGenerics && visitTrailingRequirements(TAD);
  }
  
  bool visitOpaqueTypeDecl(OpaqueTypeDecl *OTD) {
    if (Walker.shouldWalkIntoGenericParams() && OTD->getGenericParams()) {
      if (doIt(OTD->getGenericParams()))
        return true;
    }
    return false;
  }

  bool visitGenericTypeParamDecl(GenericTypeParamDecl *GTPD) {
    for (const auto &Inherit : GTPD->getInherited().getEntries()) {
      if (auto *const TyR = Inherit.getTypeRepr())
        if (doIt(TyR))
          return true;
    }
    return false;
  }

  bool visitAssociatedTypeDecl(AssociatedTypeDecl *ATD) {
    for (const auto &Inherit : ATD->getInherited().getEntries()) {
      if (auto *const TyR = Inherit.getTypeRepr())
        if (doIt(TyR))
          return true;
    }
    if (const auto DefaultTy = ATD->getDefaultDefinitionTypeRepr())
      if (doIt(DefaultTy))
        return true;

    if (auto *WhereClause = ATD->getTrailingWhereClause()) {
      for (auto &Req: WhereClause->getRequirements()) {
        if (doIt(Req))
          return true;
      }
    }
    return false;
  }

  bool visitNominalTypeDecl(NominalTypeDecl *NTD) {
#ifndef NDEBUG
    PrettyStackTraceDecl debugStack("walking into", NTD);
#endif

    bool WalkGenerics = visitGenericParamListIfNeeded(NTD);

    auto inheritedTypes = NTD->getInherited();
    for (auto i : inheritedTypes.getIndices()) {
      if (auto *const TyR = inheritedTypes.getTypeRepr(i))
        if (doIt(TyR))
          return true;
    }

    // Visit requirements
    if (WalkGenerics && visitTrailingRequirements(NTD))
      return true;

    for (Decl *Member : NTD->getMembers()) {
      if (doIt(Member))
        return true;

      if (Walker.shouldWalkAccessorsTheOldWay()) {
        // Pretend that accessors share a parent with the storage.
        //
        // FIXME: Update existing ASTWalkers to deal with accessors appearing as
        // children of the storage instead.
        if (auto *ASD = dyn_cast<AbstractStorageDecl>(Member)) {
          for (auto AD : ASD->getAllAccessors()) {
            if (doIt(AD))
              return true;
          }
        }
      }
    }
    return false;
  }

  bool visitModuleDecl(ModuleDecl *MD) {
    // TODO: should we recurse within the module?
    return false;
  }

  bool visitVarDecl(VarDecl *VD) {
    if (!Walker.shouldWalkAccessorsTheOldWay()) {
      for (auto *AD : VD->getAllAccessors())
        if (doIt(AD))
          return true;
    }

    return false;
  }

  bool visitParamDecl(ParamDecl *P) {
    // Don't walk into the type if the decl is implicit, or if the type is
    // implicit.
    if (!P->isImplicit()) {
      if (auto *repr = P->getTypeRepr()) {
        if (doIt(repr)) {
          return true;
        }
      }
    }
    if (auto *E = P->getStructuralDefaultExpr()) {
      auto res = doIt(E);
      if (!res) return true;
      P->setDefaultExpr(res, /*isTypeChecked*/ (bool)res->getType());
    }

    if (!Walker.shouldWalkAccessorsTheOldWay()) {
      for (auto *AD : P->getAllAccessors())
        if (doIt(AD))
          return true;
    }

    return false;
  }

  bool visitSubscriptDecl(SubscriptDecl *SD) {
    bool WalkGenerics = visitGenericParamListIfNeeded(SD);

    if (visit(SD->getIndices()))
      return true;

    if (auto *const TyR = SD->getElementTypeRepr())
      if (doIt(TyR))
        return true;

   // Visit trailing requirements
    if (WalkGenerics && visitTrailingRequirements(SD))
      return true;

    if (!Walker.shouldWalkAccessorsTheOldWay()) {
      for (auto *AD : SD->getAllAccessors())
        if (doIt(AD))
          return true;
    }

    return false;
  }

  bool visitMissingDecl(MissingDecl *missing) {
    return false;
  }

  bool visitMissingMemberDecl(MissingMemberDecl *MMD) {
    return false;
  }

  bool visitMacroDecl(MacroDecl *MD) {
    bool WalkGenerics = visitGenericParamListIfNeeded(MD);

    if (MD->parameterList && visit(MD->parameterList))
      return true;

    if (auto resultTypeRepr = MD->resultType.getTypeRepr()) {
      if (doIt(resultTypeRepr))
        return true;
    }

    if (auto def = MD->definition) {
      // Don't walk into unchecked definitions.
      if (auto expansion = dyn_cast<MacroExpansionExpr>(def)) {
        if (!expansion->getType().isNull() ||
            Walker.shouldWalkIntoUncheckedMacroDefinitions()) {
          if (auto newDef = doIt(def))
            MD->definition = newDef;
          else
            return true;
        }
      }
    }

    // Visit trailing requirements
    if (WalkGenerics && visitTrailingRequirements(MD))
       return true;

    return false;
  }

  bool visitFreestandingMacroArgs(FreestandingMacroExpansion *ME) {
    for (TypeRepr *genArg : ME->getGenericArgs()) {
      if (doIt(genArg))
        return true;
    }

    if (ME->getArgs()) {
      ArgumentList *args = doIt(ME->getArgs());
      if (!args)
        return true;
      ME->setArgs(args);
    }

    return false;
  }

  bool visitMacroExpansionDecl(MacroExpansionDecl *MED) {
#ifndef NDEBUG
    PrettyStackTraceDecl debugStack("walking into", MED);
#endif

    bool shouldWalkArguments, shouldWalkExpansion;
    std::tie(shouldWalkArguments, shouldWalkExpansion) =
        Walker.shouldWalkMacroArgumentsAndExpansion();

    if (shouldWalkArguments && visitFreestandingMacroArgs(MED))
      return true;

    bool alreadyFailed = false;
    if (shouldWalkExpansion) {
      MED->forEachExpandedNode([&](ASTNode expandedNode) {
        if (alreadyFailed) return;

        if (auto *expr = expandedNode.dyn_cast<Expr *>()) {
          alreadyFailed = doIt(expr) == nullptr;
        } else if (auto *stmt = expandedNode.dyn_cast<Stmt *>()) {
          alreadyFailed = doIt(stmt) == nullptr;
        } else {
          auto decl = expandedNode.get<Decl *>();
          if (!isa<VarDecl>(decl))
            alreadyFailed = doIt(decl);
        }
      });
    }
    return alreadyFailed;
  }

  bool visitAbstractFunctionDecl(AbstractFunctionDecl *AFD) {
#ifndef NDEBUG
    PrettyStackTraceDecl debugStack("walking into body of", AFD);
#endif

    bool WalkGenerics =
        // accessor generics are visited from the storage decl
        !isa<AccessorDecl>(AFD) && visitGenericParamListIfNeeded(AFD);

    if (auto *PD = AFD->getImplicitSelfDecl(/*createIfNeeded=*/false)) {
      if (visit(PD))
        return true;
    }
    if (visit(AFD->getParameters()))
      return true;

    if (auto *const ThrownTyR = AFD->getThrownTypeRepr()) {
      if (doIt(ThrownTyR))
        return true;
    }

    if (auto *FD = dyn_cast<FuncDecl>(AFD)) {
      if (!isa<AccessorDecl>(FD))
        if (auto *const TyR = FD->getResultTypeRepr())
          if (doIt(TyR))
            return true;
    }

    // Visit trailing requirements
    if (WalkGenerics && visitTrailingRequirements(AFD))
      return true;

    if (AFD->getBody(/*canSynthesize=*/false)) {
      AbstractFunctionDecl::BodyKind PreservedKind = AFD->getBodyKind();
      if (BraceStmt *S = cast_or_null<BraceStmt>(doIt(AFD->getBody())))
        AFD->setBody(S, PreservedKind);
      else
        return true;
    }

    if (auto ctor = dyn_cast<ConstructorDecl>(AFD)) {
      if (auto superInit = ctor->getSuperInitCall()) {
        if ((superInit = doIt(superInit)))
          ctor->setSuperInitCall(superInit);
        else
          return true;
      }
    }

    return false;
  }

  bool visitEnumElementDecl(EnumElementDecl *ED) {
    if (auto *PL = ED->getParameterList()) {
      if (visit(PL))
        return true;
    }

    if (auto *rawLiteralExpr = ED->getRawValueUnchecked()) {
      if (Expr *newRawExpr = doIt(rawLiteralExpr)) {
        auto *newLiteralRawExpr = cast<LiteralExpr>(newRawExpr);
        ED->setRawValueExpr(newLiteralRawExpr);
      } else {
        return true;
      }
    }
    return false;
  }

  //===--------------------------------------------------------------------===//
  //                                  Exprs
  //===--------------------------------------------------------------------===//

  // A macro for handling the "semantic expressions" that are common
  // on sugared expression nodes like string interpolation.  The
  // semantic expression is set up by type-checking to include all the
  // other children as sub-expressions, so if it exists, we should
  // just bypass the rest of the visitation.
#define HANDLE_SEMANTIC_EXPR(NODE)                         \
  do {                                                     \
    if (Expr *_semanticExpr = NODE->getSemanticExpr()) {   \
      if ((_semanticExpr = doIt(_semanticExpr))) {         \
        NODE->setSemanticExpr(_semanticExpr);              \
      } else {                                             \
        return nullptr;                                    \
      }                                                    \
      return NODE;                                         \
    }                                                      \
  } while (false)

  Expr *visitErrorExpr(ErrorExpr *E) { return E; }
  Expr *visitCodeCompletionExpr(CodeCompletionExpr *E) {
    if (Expr *baseExpr = E->getBase()) {
      Expr *newBaseExpr = doIt(baseExpr);
      if (!newBaseExpr)
        return nullptr;
      E->setBase(newBaseExpr);
    }
    return E;
  }
  Expr *visitLiteralExpr(LiteralExpr *E) { return E; }
  Expr *visitDiscardAssignmentExpr(DiscardAssignmentExpr *E) { return E; }
  Expr *visitTypeExpr(TypeExpr *E) {
    if (!E->isImplicit())
      if (auto *typerepr = E->getTypeRepr())
        if (doIt(typerepr))
          return nullptr;

    return E;
  }
  Expr *visitSuperRefExpr(SuperRefExpr *E) { return E; }
  Expr *visitOtherConstructorDeclRefExpr(OtherConstructorDeclRefExpr *E) {
    return E;
  }
  
  Expr *visitOverloadedDeclRefExpr(OverloadedDeclRefExpr *E) { return E; }
  Expr *visitUnresolvedDeclRefExpr(UnresolvedDeclRefExpr *E) { return E; }

  Expr *visitUnresolvedMemberExpr(UnresolvedMemberExpr *E) { return E; }

  Expr *visitOpaqueValueExpr(OpaqueValueExpr *E) { return E; }

  Expr *visitPropertyWrapperValuePlaceholderExpr(
      PropertyWrapperValuePlaceholderExpr *E) {
    if (E->getOpaqueValuePlaceholder()) {
      if (auto *placeholder = doIt(E->getOpaqueValuePlaceholder()))
        E->setOpaqueValuePlaceholder(dyn_cast<OpaqueValueExpr>(placeholder));
      else
        return nullptr;
    }

    if (Walker.shouldWalkIntoPropertyWrapperPlaceholderValue()) {
      if (E->getOriginalWrappedValue()) {
        if (auto *newValue = doIt(E->getOriginalWrappedValue()))
          E->setOriginalWrappedValue(newValue);
        else
          return nullptr;
      }
    }

    return E;
  }

  Expr *visitAppliedPropertyWrapperExpr(AppliedPropertyWrapperExpr *E) {
    if (auto *newValue = doIt(E->getValue())) {
      E->setValue(newValue);
    } else {
      return nullptr;
    }

    return E;
  }

  Expr *visitDefaultArgumentExpr(DefaultArgumentExpr *E) { return E; }

  Expr *visitInterpolatedStringLiteralExpr(InterpolatedStringLiteralExpr *E) {
    if (auto oldAppendingExpr = E->getAppendingExpr()) {
      if (auto appendingExpr = doIt(oldAppendingExpr))
        E->setAppendingExpr(dyn_cast<TapExpr>(appendingExpr));
      else
        return nullptr;
    }
    return E;
  }

  Expr *visitObjectLiteralExpr(ObjectLiteralExpr *E) {
    if (auto *args = doIt(E->getArgs())) {
      E->setArgs(args);
    } else {
      return nullptr;
    }
    return E;
  }

  Expr *visitCollectionExpr(CollectionExpr *E) {
    for (auto &elt : E->getElements())
      if (Expr *Sub = doIt(elt))
        elt = Sub;
      else
        return nullptr;
    return E;
  }

  Expr *visitDeclRefExpr(DeclRefExpr *E) {
    return E;
  }
  
  Expr *visitMemberRefExpr(MemberRefExpr *E) {
    if (Expr *Base = doIt(E->getBase())) {
      E->setBase(Base);
      return E;
    }
    return nullptr;
  }

  Expr *visitDynamicMemberRefExpr(DynamicMemberRefExpr *E) {
    if (Expr *Base = doIt(E->getBase())) {
      E->setBase(Base);
      return E;
    }

    return nullptr;
  }

  Expr *visitAnyTryExpr(AnyTryExpr *E) {
    if (Expr *subExpr = doIt(E->getSubExpr())) {
      E->setSubExpr(subExpr);
      return E;
    }
    return nullptr;
  }

  Expr *visitIdentityExpr(IdentityExpr *E) {
    if (Expr *subExpr = doIt(E->getSubExpr())) {
      E->setSubExpr(subExpr);
      return E;
    }
    return nullptr;
  }

  Expr *visitCopyExpr(CopyExpr *E) {
    if (Expr *subExpr = doIt(E->getSubExpr())) {
      E->setSubExpr(subExpr);
      return E;
    }
    return nullptr;
  }

  Expr *visitConsumeExpr(ConsumeExpr *E) {
    if (Expr *subExpr = doIt(E->getSubExpr())) {
      E->setSubExpr(subExpr);
      return E;
    }
    return nullptr;
  }

  Expr *visitTupleExpr(TupleExpr *E) {
    for (unsigned i = 0, e = E->getNumElements(); i != e; ++i)
      if (E->getElement(i)) {
        if (Expr *Elt = doIt(E->getElement(i)))
          E->setElement(i, Elt);
        else
          return nullptr;
      }
    return E;
  }
  Expr *visitSubscriptExpr(SubscriptExpr *E) {
    if (Expr *Base = doIt(E->getBase()))
      E->setBase(Base);
    else
      return nullptr;

    if (auto *args = doIt(E->getArgs())) {
      E->setArgs(args);
    } else {
      return nullptr;
    }
    
    return E;
  }
  Expr *visitKeyPathApplicationExpr(KeyPathApplicationExpr *E) {
    if (Expr *Base = doIt(E->getBase()))
      E->setBase(Base);
    else
      return nullptr;
    
    if (Expr *KeyPath = doIt(E->getKeyPath()))
      E->setKeyPath(KeyPath);
    else
      return nullptr;
    
    return E;
  }
  Expr *visitDynamicSubscriptExpr(DynamicSubscriptExpr *E) {
    if (Expr *Base = doIt(E->getBase()))
      E->setBase(Base);
    else
      return nullptr;

    if (auto *args = doIt(E->getArgs())) {
      E->setArgs(args);
    } else {
      return nullptr;
    }
    
    return E;
  }
  Expr *visitUnresolvedDotExpr(UnresolvedDotExpr *E) {
    if (!E->getBase())
      return E;
    
    if (Expr *E2 = doIt(E->getBase())) {
      E->setBase(E2);
      return E;
    }
    return nullptr;
  }
  Expr *visitUnresolvedSpecializeExpr(UnresolvedSpecializeExpr *E) {
    if (!E->getSubExpr())
      return E;
    
    if (Expr *Sub = doIt(E->getSubExpr()))
      E->setSubExpr(Sub);
    else
      return nullptr;

    for (auto &TyLoc : E->getUnresolvedParams()) {
      if (doIt(TyLoc))
        return nullptr;
    }

    return E;
  }
  
  Expr *visitTupleElementExpr(TupleElementExpr *E) {
    if (Expr *E2 = doIt(E->getBase())) {
      E->setBase(E2);
      return E;
    }
    return nullptr;
  }

  Expr *visitImplicitConversionExpr(ImplicitConversionExpr *E) {
    if (Expr *E2 = doIt(E->getSubExpr())) {
      E->setSubExpr(E2);
      return E;
    }
    return nullptr;
  }

  Expr *visitErasureExpr(ErasureExpr *E) {
    if (Expr *E2 = doIt(E->getSubExpr())) {
      E->setSubExpr(E2);
    } else {
      return nullptr;
    }

    for (unsigned i = 0; i < E->getArgumentConversions().size(); ++i) {
      const auto &conv = E->getArgumentConversions()[i];
      auto kConv = conv.Conversion;
      if (!kConv) {
        return nullptr;
      } else if (Expr *E2 = doIt(kConv)) {
        E->setArgumentConversion(i, {conv.OrigValue, E2});
      } else {
        return nullptr;
      }
    }
    
    return E;
  }

  Expr *visitCollectionUpcastConversionExpr(CollectionUpcastConversionExpr *E) {
    if (Expr *E2 = doIt(E->getSubExpr())) {
      E->setSubExpr(E2);
    } else {
      return nullptr;
    }

    if (auto &keyConv = E->getKeyConversion()) {
      auto kConv = keyConv.Conversion;
      if (!kConv) {
        return nullptr;
      } else if (Expr *E2 = doIt(kConv)) {
        E->setKeyConversion({keyConv.OrigValue, E2});
      } else {
        return nullptr;
      }
    }

    if (auto &valueConv = E->getValueConversion()) {
      auto vConv = valueConv.Conversion;
      if (!vConv) {
        return nullptr;
      } else if (Expr *E2 = doIt(vConv)) {
        E->setValueConversion({valueConv.OrigValue, E2});
      } else {
        return nullptr;
      }
    }

    return E;
  }
  
  Expr *visitDestructureTupleExpr(DestructureTupleExpr *E) {
    if (auto *src = doIt(E->getSubExpr())) {
      E->setSubExpr(src);
    } else {
      return nullptr;
    }

    if (auto *dst = doIt(E->getResultExpr())) {
      E->setResultExpr(dst);
    } else {
      return nullptr;
    }

    return E;
  }

  Expr *visitTryExpr(TryExpr *E) {
    if (Expr *E2 = doIt(E->getSubExpr())) {
      E->setSubExpr(E2);
      return E;
    }
    return nullptr;
  }

  Expr *visitForceTryExpr(ForceTryExpr *E) {
    if (Expr *E2 = doIt(E->getSubExpr())) {
      E->setSubExpr(E2);
      return E;
    }
    return nullptr;
  }

  Expr *visitOptionalTryExpr(OptionalTryExpr *E) {
    if (Expr *E2 = doIt(E->getSubExpr())) {
      E->setSubExpr(E2);
      return E;
    }
    return nullptr;
  }

  Expr *visitInOutExpr(InOutExpr *E) {
    if (Expr *E2 = doIt(E->getSubExpr())) {
      E->setSubExpr(E2);
      return E;
    }
    return nullptr;
  }
  
  Expr *visitVarargExpansionExpr(VarargExpansionExpr *E) {
    if (Expr *E2 = doIt(E->getSubExpr())) {
      E->setSubExpr(E2);
      return E;
    }
    return nullptr;
  }

  Expr *visitPackExpansionExpr(PackExpansionExpr *E) {
    if (Expr *pattern = doIt(E->getPatternExpr())) {
      E->setPatternExpr(pattern);
      return E;
    }
    return nullptr;
  }

  Expr *visitPackElementExpr(PackElementExpr *E) {
    if (Expr *pattern = doIt(E->getPackRefExpr())) {
      E->setPackRefExpr(pattern);
      return E;
    }
    return nullptr;
  }

  Expr *visitMaterializePackExpr(MaterializePackExpr *E) {
    if (Expr *fromExpr = doIt(E->getFromExpr())) {
      E->setFromExpr(fromExpr);
      return E;
    }
    return nullptr;
  }

  Expr *visitSequenceExpr(SequenceExpr *E) {
    for (unsigned i = 0, e = E->getNumElements(); i != e; ++i)
      if (Expr *Elt = doIt(E->getElement(i)))
        E->setElement(i, Elt);
      else
        return nullptr;
    return E;
  }

  Expr *visitDynamicTypeExpr(DynamicTypeExpr *E) {
    Expr *base = E->getBase();
    if ((base = doIt(base)))
      E->setBase(base);
    else
      return nullptr;

    return E;
  }

  Expr *visitCaptureListExpr(CaptureListExpr *expr) {
    for (auto c : expr->getCaptureList()) {
      if (Walker.shouldWalkCaptureInitializerExpressions()) {
        for (auto entryIdx : range(c.PBD->getNumPatternEntries())) {
          if (auto newInit = doIt(c.PBD->getInit(entryIdx)))
            c.PBD->setInit(entryIdx, newInit);
          else
            return nullptr;
        }
      } else {
        if (doIt(c.PBD))
          return nullptr;
      }
    }

    AbstractClosureExpr *body = expr->getClosureBody();
    if ((body = cast_or_null<AbstractClosureExpr>(doIt(body))))
      expr->setClosureBody(body);
    else
      return nullptr;
    return expr;
  }

  Expr *visitClosureExpr(ClosureExpr *expr) {
    if (visit(expr->getParameters()))
      return nullptr;

    if (auto thrownTypeRepr = expr->getExplicitThrownTypeRepr()) {
      if (doIt(thrownTypeRepr))
        return nullptr;
    }

    if (expr->hasExplicitResultType()) {
      if (doIt(expr->getExplicitResultTypeRepr()))
        return nullptr;
    }

    // If the closure was separately type checked and we don't want to
    // visit separately-checked closure bodies, bail out now.
    if (expr->isSeparatelyTypeChecked() &&
        !Walker.shouldWalkIntoSeparatelyCheckedClosure(expr))
      return expr;

    // Handle other closures.
    if (BraceStmt *body = cast_or_null<BraceStmt>(doIt(expr->getBody()))) {
      expr->setBody(body, expr->hasSingleExpressionBody());
      return expr;
    }
    return nullptr;
  }

  Expr *visitAutoClosureExpr(AutoClosureExpr *E) {
    if (Expr *E2 = doIt(E->getSingleExpressionBody())) {
      E->setBody(E2);
      return E;
    }
    return nullptr;
  }
  
  Expr *visitApplyExpr(ApplyExpr *E) {
    if (E->getFn()) {
      Expr *E2 = doIt(E->getFn());
      if (E2 == nullptr) return nullptr;
      E->setFn(E2);
    }

    if (auto *args = doIt(E->getArgs())) {
      E->setArgs(args);
    } else {
      return nullptr;
    }

    return E;
  }

  Expr *visitSelfApplyExpr(SelfApplyExpr *E) {
    if (E->getBase()) {
      Expr *E2 = doIt(E->getBase());
      if (E2 == nullptr) return nullptr;
      E->setBase(E2);
    }

    if (E->getFn()) {
      Expr *E2 = doIt(E->getFn());
      if (E2 == nullptr) return nullptr;
      E->setFn(E2);
    }

    return E;
  }

  Expr *visitDotSyntaxBaseIgnoredExpr(DotSyntaxBaseIgnoredExpr *E) {
    Expr *E2 = doIt(E->getLHS());
    if (E2 == nullptr) return nullptr;
    E->setLHS(E2);
    
    E2 = doIt(E->getRHS());
    if (E2 == nullptr) return nullptr;
    E->setRHS(E2);
    return E;      
  }

  Expr *visitExplicitCastExpr(ExplicitCastExpr *E) {
    if (Expr *Sub = E->getSubExpr()) {
      Sub = doIt(Sub);
      if (!Sub) return nullptr;
      E->setSubExpr(Sub);
    }

    if (auto *const tyRepr = E->getCastTypeRepr())
      if (doIt(tyRepr))
        return nullptr;

    return E;
  }

  Expr *visitArrowExpr(ArrowExpr *E) {
    if (Expr *Args = E->getArgsExpr()) {
      Args = doIt(Args);
      if (!Args) return nullptr;
      E->setArgsExpr(Args);
    }
    if (Expr *Thrown = E->getThrownTypeExpr()) {
      Thrown = doIt(Thrown);
      if (!Thrown) return nullptr;
      E->setThrownTypeExpr(Thrown);
    }
    if (Expr *Result = E->getResultExpr()) {
      Result = doIt(Result);
      if (!Result) return nullptr;
      E->setResultExpr(Result);
    }
    return E;
  }

  Expr *visitRebindSelfInConstructorExpr(RebindSelfInConstructorExpr *E) {
    Expr *Sub = doIt(E->getSubExpr());
    if (!Sub) return nullptr;
    E->setSubExpr(Sub);
    
    return E;
  }
  
  Expr *visitAssignExpr(AssignExpr *AE) {
    if (Expr *Dest = AE->getDest()) {
      if (!(Dest = doIt(Dest)))
        return nullptr;
      AE->setDest(Dest);
    }

    if (Expr *Src = AE->getSrc()) {
      if (!(Src = doIt(AE->getSrc())))
        return nullptr;
      AE->setSrc(Src);
    }
    
    return AE;
  }
  
  Expr *visitEnumIsCaseExpr(EnumIsCaseExpr *E) {
    if (Expr *Sub = E->getSubExpr()) {
      if (!(Sub = doIt(Sub)))
        return nullptr;
      E->setSubExpr(Sub);
    }

    if (auto *typerepr = E->getCaseTypeRepr())
      if (doIt(typerepr))
        return nullptr;

    return E;
  }

  Expr *visitTernaryExpr(TernaryExpr *E) {
    if (Expr *Cond = E->getCondExpr()) {
      Cond = doIt(Cond);
      if (!Cond) return nullptr;
      E->setCondExpr(Cond);
    }
    
    Expr *Then = doIt(E->getThenExpr());
    if (!Then) return nullptr;
    E->setThenExpr(Then);
    
    if (Expr *Else = E->getElseExpr()) {
      Else = doIt(Else);
      if (!Else) return nullptr;
      E->setElseExpr(Else);
    }
    
    return E;
  }

  Expr *visitUnresolvedPatternExpr(UnresolvedPatternExpr *E) {
    Pattern *sub = doIt(E->getSubPattern());
    if (!sub) return nullptr;
    
    E->setSubPattern(sub);
    return E;
  }

  Expr *visitBindOptionalExpr(BindOptionalExpr *E) {
    Expr *sub = doIt(E->getSubExpr());
    if (!sub) return nullptr;

    E->setSubExpr(sub);
    return E;
  }

  Expr *visitOptionalEvaluationExpr(OptionalEvaluationExpr *E) {
    Expr *sub = doIt(E->getSubExpr());
    if (!sub) return nullptr;

    E->setSubExpr(sub);
    return E;
  }

  Expr *visitForceValueExpr(ForceValueExpr *E) {
    Expr *sub = doIt(E->getSubExpr());
    if (!sub) return nullptr;

    E->setSubExpr(sub);
    return E;
  }

  Expr *visitOpenExistentialExpr(OpenExistentialExpr *E) {
    Expr *existential = doIt(E->getExistentialValue());
    if (!existential) return nullptr;

    Expr *sub = doIt(E->getSubExpr());
    if (!sub) return nullptr;

    E->setExistentialValue(existential);
    E->setSubExpr(sub);
    return E;
  }

  Expr *visitMakeTemporarilyEscapableExpr(MakeTemporarilyEscapableExpr *E) {
    Expr *closure = doIt(E->getNonescapingClosureValue());
    if (!closure) return nullptr;

    Expr *sub = doIt(E->getSubExpr());
    if (!sub) return nullptr;

    E->setNonescapingClosureValue(closure);
    E->setSubExpr(sub);
    return E;
  }
  
  Expr *visitEditorPlaceholderExpr(EditorPlaceholderExpr *E) {
    HANDLE_SEMANTIC_EXPR(E);
    return E;
  }

  Expr *visitLazyInitializerExpr(LazyInitializerExpr *E) {
    // The initializer is opaque unless we specifically want to visit it as part
    // of the accessor body.
    if (Walker.getLazyInitializerWalkingBehavior() !=
        LazyInitializerWalking::InAccessor) {
      return E;
    }
    auto *sub = doIt(E->getSubExpr());
    if (!sub)
      return nullptr;

    E->setSubExpr(sub);
    return E;
  }

  Expr *visitObjCSelectorExpr(ObjCSelectorExpr *E) {
    Expr *sub = doIt(E->getSubExpr());
    if (!sub) return nullptr;

    E->setSubExpr(sub);
    return E;
  }

  Expr *visitKeyPathExpr(KeyPathExpr *E) {
    // For an ObjC key path, the string literal expr serves as the semantic
    // expression.
    if (auto objcStringLiteral = E->getObjCStringLiteralExpr()) {
      Expr *sub = doIt(objcStringLiteral);
      if (!sub) return nullptr;
      E->setObjCStringLiteralExpr(sub);
    }

    auto components = E->getComponents();
    if (components.empty()) {
      // No components means a parsed-only/pre-resolution Swift key path.
      assert(!E->isObjC());
      if (auto parsedRoot = E->getParsedRoot()) {
        Expr *newRoot = doIt(parsedRoot);
        if (!newRoot)
          return nullptr;
        E->setParsedRoot(newRoot);
      }
      if (auto parsedPath = E->getParsedPath()) {
        Expr *newPath = doIt(parsedPath);
        if (!newPath)
          return nullptr;
        E->setParsedPath(newPath);
      }
      return E;
    }

    if (!E->isObjC()) {
      auto rootType = E->getExplicitRootType();
      if (rootType && doIt(rootType))
        return nullptr;
    }

    for (auto &origComponent : components) {
      auto component = origComponent;
      switch (auto kind = component.getKind()) {
      case KeyPathExpr::Component::Kind::Subscript:
      case KeyPathExpr::Component::Kind::UnresolvedSubscript: {
        if (auto *newArgs = doIt(component.getSubscriptArgs())) {
          component.setSubscriptArgs(newArgs);
        } else {
          return nullptr;
        }
        break;
      }
        
      case KeyPathExpr::Component::Kind::OptionalChain:
      case KeyPathExpr::Component::Kind::OptionalWrap:
      case KeyPathExpr::Component::Kind::OptionalForce:
      case KeyPathExpr::Component::Kind::Property:
      case KeyPathExpr::Component::Kind::UnresolvedProperty:
      case KeyPathExpr::Component::Kind::Invalid:
      case KeyPathExpr::Component::Kind::Identity:
      case KeyPathExpr::Component::Kind::TupleElement:
      case KeyPathExpr::Component::Kind::DictionaryKey:
      case KeyPathExpr::Component::Kind::CodeCompletion:
        // No subexpr to visit.
        break;
      }
    }
    return E;
  }

  Expr *visitKeyPathDotExpr(KeyPathDotExpr *E) { return E; }

  Expr *visitSingleValueStmtExpr(SingleValueStmtExpr *E) {
    if (auto *S = doIt(E->getStmt())) {
      E->setStmt(S);
    } else {
      return nullptr;
    }
    return E;
  }

  Expr *visitOneWayExpr(OneWayExpr *E) {
    if (auto oldSubExpr = E->getSubExpr()) {
      if (auto subExpr = doIt(oldSubExpr)) {
        E->setSubExpr(subExpr);
      } else {
        return nullptr;
      }
    }

    return E;
  }

  Expr *visitTapExpr(TapExpr *E) {
    if (auto oldSubExpr = E->getSubExpr()) {
      if (auto subExpr = doIt(oldSubExpr)) {
        E->setSubExpr(subExpr);
      } else {
        return nullptr;
      }
    }

    if (!Walker.shouldWalkIntoTapExpression())
      return E;

    if (auto oldBody = E->getBody()) {
      if (auto body = doIt(oldBody)) {
        E->setBody(dyn_cast<BraceStmt>(body));
      }
      else {
        return nullptr;
      }
    }

    return E;
  }

  Expr *visitRegexLiteralExpr(RegexLiteralExpr *E) {
    return E;
  }

  Expr *visitTypeJoinExpr(TypeJoinExpr *E) {
    if (auto *var = E->getVar()) {
      if (auto *newVar = dyn_cast<DeclRefExpr>(doIt(var))) {
        E->setVar(newVar);
      } else {
        return nullptr;
      }
    }

    for (unsigned i = 0, e = E->getNumElements(); i != e; ++i) {
      if (auto *origElt = E->getElement(i)) {
        if (Expr *Elt = doIt(origElt))
          E->setElement(i, Elt);
        else
          return nullptr;
      }
    }

    return E;
  }

  Expr *visitMacroExpansionExpr(MacroExpansionExpr *E) {
    bool shouldWalkArguments, shouldWalkExpansion;
    std::tie(shouldWalkArguments, shouldWalkExpansion) =
        Walker.shouldWalkMacroArgumentsAndExpansion();

    if (auto *substituteDecl = E->getSubstituteDecl()) {
      if (doIt(substituteDecl))
        return nullptr;
      // Visiting the substitute macro expansion decl will visit the same
      // argument list. Skip visiting it again.
      shouldWalkArguments = false;
    }

    if (shouldWalkArguments && visitFreestandingMacroArgs(E))
      return nullptr;

    if (shouldWalkExpansion) {
      Expr *rewritten = nullptr;
      if (E->getRewritten()) {
        rewritten = doIt(E->getRewritten());
        if (!rewritten) return nullptr;
      }
      E->setRewritten(rewritten);
    }
    return E;
  }

  //===--------------------------------------------------------------------===//
  //                           Everything Else
  //===--------------------------------------------------------------------===//

#define STMT(Id, Parent) Stmt *visit##Id##Stmt(Id##Stmt *S);
#include "swift/AST/StmtNodes.def"

#define PATTERN(Id, Parent) Pattern *visit##Id##Pattern(Id##Pattern *P);
#include "swift/AST/PatternNodes.def"

#define TYPEREPR(Id, Parent) bool visit##Id##TypeRepr(Id##TypeRepr *T);
#include "swift/AST/TypeReprNodes.def"

  using Action = ASTWalker::Action;

  using PreWalkAction = ASTWalker::PreWalkAction;
  using PostWalkAction = ASTWalker::PostWalkAction;

  template <typename T>
  using PreWalkResult = ASTWalker::PreWalkResult<T>;

  template <typename T>
  using PostWalkResult = ASTWalker::PostWalkResult<T>;

  [[nodiscard]]
  bool traverse(PreWalkAction Pre, llvm::function_ref<bool(void)> VisitChildren,
                llvm::function_ref<PostWalkAction(void)> WalkPost) {
    switch (Pre.Action) {
    case PreWalkAction::Stop:
      return true;
    case PreWalkAction::SkipChildren:
      return false;
    case PreWalkAction::Continue:
      break;
    }
    if (VisitChildren())
      return true;
    switch (WalkPost().Action) {
    case PostWalkAction::Stop:
      return true;
    case PostWalkAction::Continue:
      return false;
    }
    llvm_unreachable("Unhandled case in switch!");
  }

  template <typename T>
  [[nodiscard]]
  T *traverse(PreWalkResult<T *> Pre,
              llvm::function_ref<T *(T *)> VisitChildren,
              llvm::function_ref<PostWalkResult<T *>(T *)> WalkPost) {
    switch (Pre.Action.Action) {
    case PreWalkAction::Stop:
      return nullptr;
    case PreWalkAction::SkipChildren:
      assert(*Pre.Value && "Use Action::Stop instead of returning nullptr");
      return *Pre.Value;
    case PreWalkAction::Continue:
      break;
    }
    assert(*Pre.Value && "Use Action::Stop instead of returning nullptr");
    auto Value = VisitChildren(*Pre.Value);
    if (!Value)
      return nullptr;

    auto Post = WalkPost(Value);
    switch (Post.Action.Action) {
    case PostWalkAction::Stop:
      return nullptr;
    case PostWalkAction::Continue:
      assert(*Post.Value && "Use Action::Stop instead of returning nullptr");
      return *Post.Value;
    }
    llvm_unreachable("Unhandled case in switch!");
  }

  [[nodiscard]]
  bool visitParameterList(ParameterList *PL) {
    return traverse(
        Walker.walkToParameterListPre(PL),
        [&]() {
          for (auto P : *PL) {
            // Walk each parameter's decl and typeloc and default value.
            if (doIt(P))
              return true;
          }
          return false;
        },
        [&]() { return Walker.walkToParameterListPost(PL); });
  }

public:
  Traversal(ASTWalker &walker) : Walker(walker) {}

  [[nodiscard]]
  Expr *doIt(Expr *E) {
    return traverse<Expr>(
        Walker.walkToExprPre(E),
        [&](Expr *E) { return visit(E); },
        [&](Expr *E) { return Walker.walkToExprPost(E); });
  }

  [[nodiscard]]
  Stmt *doIt(Stmt *S) {
    return traverse<Stmt>(
        Walker.walkToStmtPre(S),
        [&](Stmt *S) { return visit(S); },
        [&](Stmt *S) { return Walker.walkToStmtPost(S); });
  }
  
  bool shouldSkip(Decl *D) {
    if (!Walker.shouldWalkMacroArgumentsAndExpansion().second &&
        D->isInMacroExpansionInContext() && !Walker.Parent.isNull())
      return true;

    if (auto *VD = dyn_cast<VarDecl>(D)) {
      // VarDecls are walked via their NamedPattern, ignore them if we encounter
      // then in the few cases where they are also pushed outside as members.
      // In all those cases we can walk them via the pattern binding decl.
      // This is used for when vising VarDecls from source, when visiting a
      // module file we walk them as we encounter them.
      if (Walker.Parent.getAsModule() &&
          D->getDeclContext()->getParentSourceFile())
        return true;
      if (Walker.Parent.getAsDecl() && VD->getParentPatternBinding())
        return true;
      auto walkerParentAsStmt = Walker.Parent.getAsStmt();
      if (isa_and_nonnull<BraceStmt>(walkerParentAsStmt))
        return true;
    }
    return false;
  }

  /// Returns true on failure.
  [[nodiscard]]
  bool doIt(Decl *D) {
    if (shouldSkip(D))
      return false;

    return traverse(
        Walker.walkToDeclPre(D),
        [&]() { return visit(D); },
        [&]() { return Walker.walkToDeclPost(D); });
  }

  [[nodiscard]]
  Pattern *doIt(Pattern *P) {
    return traverse<Pattern>(
        Walker.walkToPatternPre(P),
        [&](Pattern *P) { return visit(P); },
        [&](Pattern *P) { return Walker.walkToPatternPost(P); });
  }

  [[nodiscard]]
  bool doIt(const StmtCondition &C) {
    for (auto &elt : C) {
      switch (elt.getKind()) {
      case StmtConditionElement::CK_Availability:
        break;
      case StmtConditionElement::CK_HasSymbol: {
        auto E = elt.getHasSymbolInfo()->getSymbolExpr();
        if (!E)
          return true;
        E = doIt(E);
        if (!E)
          return true;
        elt.getHasSymbolInfo()->setSymbolExpr(E);
        break;
      }
      case StmtConditionElement::CK_Boolean: {
        auto E = elt.getBoolean();
        // Walk an expression condition normally.
        E = doIt(E);
        if (!E)
          return true;
        elt.setBoolean(E);
        break;
      }
          
      case StmtConditionElement::CK_PatternBinding: {
        auto *P = doIt(elt.getPattern());
        if (!P) return true;
        elt.setPattern(P);

        auto *I = doIt(elt.getInitializer());
        if (!I) return true;
        elt.setInitializer(I);
      }
      }
    }
    return false;
  }

  /// Returns true on failure.
  [[nodiscard]]
  bool doIt(TypeRepr *T) {
    return traverse(
        Walker.walkToTypeReprPre(T),
        [&]() { return visit(T); },
        [&]() { return Walker.walkToTypeReprPost(T); });
  }

  [[nodiscard]]
  bool doIt(RequirementRepr &Req) {
    switch (Req.getKind()) {
    case RequirementReprKind::SameType:
      if (doIt(Req.getFirstTypeRepr()) || doIt(Req.getSecondTypeRepr()))
        return true;
      break;
    case RequirementReprKind::TypeConstraint:
      if (doIt(Req.getSubjectRepr()) || doIt(Req.getConstraintRepr()))
        return true;
      break;
    case RequirementReprKind::LayoutConstraint:
      if (doIt(Req.getSubjectRepr()))
        return true;
      break;
    }
    return false;
  }

  [[nodiscard]]
  bool doIt(GenericParamList *GPL) {
    // Visit generic params
    for (auto &P : GPL->getParams()) {
      if (doIt(P))
        return true;
    }

    // Visit param conformance
    for (auto Req : GPL->getRequirements()) {
      if (doIt(Req))
        return true;
    }

    return false;
  }

  [[nodiscard]]
  bool doIt(ArgumentList *ArgList, unsigned Idx) {
    auto Arg = ArgList->get(Idx);
    return traverse(
        Walker.walkToArgumentPre(Arg),
        [&]() {
          auto *E = doIt(Arg.getExpr());
          if (!E)
            return true;
          ArgList->setExpr(Idx, E);
          return false;
        },
        [&]() { return Walker.walkToArgumentPost(Arg); });
  }

  [[nodiscard]]
  ArgumentList *visit(ArgumentList *ArgList) {
    for (auto Idx : indices(*ArgList)) {
      if (doIt(ArgList, Idx))
        return nullptr;
    }
    return ArgList;
  }

  [[nodiscard]]
  ArgumentList *doIt(ArgumentList *ArgList) {
    return traverse<ArgumentList>(
        Walker.walkToArgumentListPre(ArgList),
        [&](ArgumentList *ArgList) { return visit(ArgList); },
        [&](ArgumentList *ArgList) {
          return Walker.walkToArgumentListPost(ArgList);
        });
  }
};

} // end anonymous namespace

#pragma mark Statement traversal
Stmt *Traversal::visitBreakStmt(BreakStmt *BS) {
  return BS;
}

Stmt *Traversal::visitContinueStmt(ContinueStmt *CS) {
  return CS;
}

Stmt *Traversal::visitFallthroughStmt(FallthroughStmt *CS) {
  return CS;
}

Stmt *Traversal::visitFailStmt(FailStmt *FS) {
  return FS;
}
Stmt *Traversal::visitThrowStmt(ThrowStmt *TS) {
  if (Expr *E = doIt(TS->getSubExpr())) {
    TS->setSubExpr(E);
    return TS;
  }
  return nullptr;
}

Stmt *Traversal::visitDiscardStmt(DiscardStmt *DS) {
  if (Expr *E = doIt(DS->getSubExpr())) {
    DS->setSubExpr(E);
    return DS;
  }
  return nullptr;
}

Stmt *Traversal::visitPoundAssertStmt(PoundAssertStmt *S) {
  if (auto *condition = doIt(S->getCondition())) {
    S->setCondition(condition);
  } else {
    return nullptr;
  }
  return S;
}

Stmt *Traversal::visitBraceStmt(BraceStmt *BS) {
  for (auto &Elem : BS->getElements()) {
    if (auto *SubExpr = Elem.dyn_cast<Expr*>()) {
      if (Expr *E2 = doIt(SubExpr))
        Elem = E2;
      else
        return nullptr;
      continue;
    }

    if (auto *S = Elem.dyn_cast<Stmt*>()) {
      if (Stmt *S2 = doIt(S))
        Elem = S2;
      else
        return nullptr;
      continue;
    }

    auto *D = Elem.get<Decl*>();
    if (doIt(D))
      return nullptr;

    if (Walker.shouldWalkAccessorsTheOldWay()) {
      // Pretend that accessors share a parent with the storage.
      //
      // FIXME: Update existing ASTWalkers to deal with accessors appearing as
      // children of the storage instead.
      if (auto *ASD = dyn_cast<AbstractStorageDecl>(D)) {
        for (auto AD : ASD->getAllAccessors()) {
          if (doIt(AD))
            return nullptr;
        }
      }
    }
  }

  return BS;
}

Stmt *Traversal::visitReturnStmt(ReturnStmt *RS) {
  if (!RS->hasResult())
    return RS;
  if (Expr *E = doIt(RS->getResult()))
    RS->setResult(E);
  else
    return nullptr;
  return RS;
}

Stmt *Traversal::visitYieldStmt(YieldStmt *YS) {
  for (auto &yield : YS->getMutableYields()) {
    if (Expr *E = doIt(yield))
      yield = E;
    else
      return nullptr;
  }
  return YS;
}

Stmt *Traversal::visitThenStmt(ThenStmt *TS) {
  auto *E = doIt(TS->getResult());
  if (!E)
    return nullptr;

  TS->setResult(E);
  return TS;
}

Stmt *Traversal::visitDeferStmt(DeferStmt *DS) {
  if (doIt(DS->getTempDecl()))
    return nullptr;

  if (Expr *Call = doIt(DS->getCallExpr()))
    DS->setCallExpr(Call);
  else
    return nullptr;
  return DS;
}

Stmt *Traversal::visitIfStmt(IfStmt *IS) {
  if (doIt(IS->getCond()))
    return nullptr;

  if (auto *S2 = doIt(IS->getThenStmt()))
    IS->setThenStmt(cast<BraceStmt>(S2));
  else
    return nullptr;

  if (IS->getElseStmt()) {
    if (Stmt *S2 = doIt(IS->getElseStmt()))
      IS->setElseStmt(S2);
    else
      return nullptr;
  }
  return IS;
}

Stmt *Traversal::visitGuardStmt(GuardStmt *US) {
  if (doIt(US->getCond()))
    return nullptr;
  
  if (BraceStmt *S2 = cast_or_null<BraceStmt>(doIt(US->getBody())))
    US->setBody(S2);
  else
    return nullptr;
  return US;
}

Stmt *Traversal::visitDoStmt(DoStmt *DS) {
  if (BraceStmt *S2 = cast_or_null<BraceStmt>(doIt(DS->getBody())))
    DS->setBody(S2);
  else
    return nullptr;

  return DS;
}

Stmt *Traversal::visitDoCatchStmt(DoCatchStmt *stmt) {
  // Transform the body of the 'do'.
  if (Stmt *newBody = doIt(stmt->getBody())) {
    stmt->setBody(newBody);
  } else {
    return nullptr;
  }

  // Transform each of the catch clauses:
  for (CaseStmt *&clause : stmt->getMutableCatches()) {
    if (auto newClause = doIt(clause)) {
      clause = cast<CaseStmt>(newClause);
    } else {
      return nullptr;
    }
  }

  return stmt;
}

Stmt *Traversal::visitWhileStmt(WhileStmt *WS) {
  if (doIt(WS->getCond()))
    return nullptr;

  if (Stmt *S2 = doIt(WS->getBody()))
    WS->setBody(S2);
  else
    return nullptr;
  return WS;
}

Stmt *Traversal::visitRepeatWhileStmt(RepeatWhileStmt *RWS) {
  if (Stmt *S2 = doIt(RWS->getBody()))
    RWS->setBody(S2);
  else
    return nullptr;

  if (Expr *E2 = doIt(RWS->getCond()))
    RWS->setCond(E2);
  else
    return nullptr;

  return RWS;
}

Stmt *Traversal::visitForEachStmt(ForEachStmt *S) {
  if (Pattern *P = S->getPattern()) {
    if ((P = doIt(P)))
      assert(P == S->getPattern() && "cannot change pattern of ForEachStmt");
    else
      return nullptr;
  }

  // The iterator decl is built directly on top of the sequence
  // expression, so don't visit both.
  //
  // If for-in is already type-checked, the type-checked version
  // of the sequence is going to be visited as part of `iteratorVar`.
  if (auto IteratorVar = S->getIteratorVar()) {
    if (doIt(IteratorVar))
      return nullptr;

    if (auto NextCall = S->getNextCall()) {
      if ((NextCall = doIt(NextCall)))
        S->setNextCall(NextCall);
      else
        return nullptr;
    }
  } else {
    if (Expr *Sequence = S->getParsedSequence()) {
      if ((Sequence = doIt(Sequence)))
        S->setParsedSequence(Sequence);
      else
        return nullptr;
    }
  }

  if (Expr *Where = S->getWhere()) {
    if ((Where = doIt(Where)))
      S->setWhere(Where);
    else
      return nullptr;
  }

  if (auto IteratorNext = S->getConvertElementExpr()) {
    if ((IteratorNext = doIt(IteratorNext)))
      S->setConvertElementExpr(IteratorNext);
    else
      return nullptr;
  }

  if (Stmt *Body = S->getBody()) {
    if ((Body = doIt(Body)))
      S->setBody(cast<BraceStmt>(Body));
    else
      return nullptr;
  }

  return S;
}

Stmt *Traversal::visitSwitchStmt(SwitchStmt *S) {
  if (Expr *newSubject = doIt(S->getSubjectExpr()))
    S->setSubjectExpr(newSubject);
  else
    return nullptr;

  for (auto N : S->getRawCases()) {
    if (Stmt *aCase = N.dyn_cast<Stmt*>()) {
      assert(isa<CaseStmt>(aCase));
      if (Stmt *aStmt = doIt(aCase)) {
        assert(aCase == aStmt && "switch case remap not supported");
        (void)aStmt;
      } else
        return nullptr;
    } else {
      assert(isa<IfConfigDecl>(N.get<Decl*>()) || 
             isa<PoundDiagnosticDecl>(N.get<Decl*>()));
      if (doIt(N.get<Decl*>()))
        return nullptr;
    }
  }

  return S;
}

Stmt *Traversal::visitCaseStmt(CaseStmt *S) {
  for (auto &CLI : S->getMutableCaseLabelItems()) {
    if (auto *newPattern = doIt(CLI.getPattern()))
      CLI.setPattern(newPattern, /*resolved=*/CLI.isPatternResolved());
    else
      return nullptr;
    if (CLI.getGuardExpr()) {
      if (auto *newGuard = doIt(CLI.getGuardExpr()))
        CLI.setGuardExpr(newGuard);
      else
        return nullptr;
    }
  }

  if (BraceStmt *newBody = cast_or_null<BraceStmt>(doIt(S->getBody())))
    S->setBody(newBody);
  else
    return nullptr;

  return S;
}

#pragma mark Pattern traversal
Pattern *Traversal::visitParenPattern(ParenPattern *P) {
  if (Pattern *newSub = doIt(P->getSubPattern()))
    P->setSubPattern(newSub);
  else
    return nullptr;
  return P;
}

Pattern *Traversal::visitTuplePattern(TuplePattern *P) {
  for (auto &element : P->getElements()) {
    if (Pattern *newField = doIt(element.getPattern()))
      element.setPattern(newField);
    else
      return nullptr;
  }
  return P;
}

Pattern *Traversal::visitNamedPattern(NamedPattern *P) {
  if (doIt(P->getDecl()))
    return nullptr;
  return P;
}

Pattern *Traversal::visitAnyPattern(AnyPattern *P) {
  return P;
}

Pattern *Traversal::visitTypedPattern(TypedPattern *P) {
  if (Pattern *newSub = doIt(P->getSubPattern()))
    P->setSubPattern(newSub);
  else
    return nullptr;
  if (!P->isImplicit())
    if (auto *TR = P->getTypeRepr())
      if (doIt(TR))
        return nullptr;
  return P;
}

Pattern *Traversal::visitIsPattern(IsPattern *P) {
  if (auto sub = P->getSubPattern()) {
    if (Pattern *newSub = doIt(sub)) {
      P->setSubPattern(newSub);
    } else {
      return nullptr;
    }
  }
  if (!P->isImplicit())
    if (auto *TR = P->getCastTypeRepr())
      if (doIt(TR))
        return nullptr;
  return P;
}

Pattern *Traversal::visitEnumElementPattern(EnumElementPattern *P) {
  if (auto *TR = P->getParentTypeRepr())
    if (doIt(TR))
      return nullptr;

  if (!P->hasSubPattern())
    return P;

  if (Pattern *newSub = doIt(P->getSubPattern())) {
    P->setSubPattern(newSub);
    return P;
  }
  return nullptr;
}

Pattern *Traversal::visitExprPattern(ExprPattern *P) {
  // If the pattern has been type-checked, walk the match expression, which
  // includes the explicit subexpression.
  if (auto *match = P->getCachedMatchExpr()) {
    if (Expr *newMatch = doIt(match)) {
      P->setMatchExpr(newMatch);
      return P;
    }
    return nullptr;
  }

  if (Expr *newSub = doIt(P->getSubExpr())) {
    P->setSubExpr(newSub);
    return P;
  }
  return nullptr;
}

Pattern *Traversal::visitBindingPattern(BindingPattern *P) {
  if (Pattern *newSub = doIt(P->getSubPattern())) {
    P->setSubPattern(newSub);
    return P;
  }
  return nullptr;
}

Pattern *Traversal::visitOptionalSomePattern(OptionalSomePattern *P) {
  if (Pattern *newSub = doIt(P->getSubPattern())) {
    P->setSubPattern(newSub);
    return P;
  }
  return nullptr;
}

Pattern *Traversal::visitBoolPattern(BoolPattern *P) {
  return P;
}

#pragma mark Type representation traversal
bool Traversal::visitErrorTypeRepr(ErrorTypeRepr *T) {
  return false;
}

bool Traversal::visitAttributedTypeRepr(AttributedTypeRepr *T) {
  return doIt(T->getTypeRepr());
}

bool Traversal::visitSimpleIdentTypeRepr(SimpleIdentTypeRepr *T) {
  return false;
}

bool Traversal::visitGenericIdentTypeRepr(GenericIdentTypeRepr *T) {
  for (auto genArg : T->getGenericArgs()) {
    if (doIt(genArg))
      return true;
  }
  return false;
}

bool Traversal::visitMemberTypeRepr(MemberTypeRepr *T) {
  if (doIt(T->getBaseComponent()))
    return true;

  for (auto comp : T->getMemberComponents()) {
    if (doIt(comp))
      return true;
  }
  return false;
}

bool Traversal::visitFunctionTypeRepr(FunctionTypeRepr *T) {
  return doIt(T->getArgsTypeRepr()) || doIt(T->getResultTypeRepr());
}

bool Traversal::visitArrayTypeRepr(ArrayTypeRepr *T) {
  return doIt(T->getBase());
}

bool Traversal::visitDictionaryTypeRepr(DictionaryTypeRepr *T) {
  return doIt(T->getKey()) || doIt(T->getValue());
}

bool Traversal::visitOptionalTypeRepr(OptionalTypeRepr *T) {
  return doIt(T->getBase());
}

bool Traversal::visitImplicitlyUnwrappedOptionalTypeRepr(ImplicitlyUnwrappedOptionalTypeRepr *T) {
  return doIt(T->getBase());
}

bool Traversal::visitVarargTypeRepr(VarargTypeRepr *T) {
  return doIt(T->getElementType());
}

bool Traversal::visitPackTypeRepr(PackTypeRepr *T) {
  for (auto &elem : T->getMutableElements())
    if (doIt(elem))
      return true;
  return false;
}

bool Traversal::visitPackExpansionTypeRepr(PackExpansionTypeRepr *T) {
  return doIt(T->getPatternType());
}

bool Traversal::visitPackElementTypeRepr(PackElementTypeRepr *T) {
  return doIt(T->getPackType());
}

bool Traversal::visitTupleTypeRepr(TupleTypeRepr *T) {
  for (auto &elem : T->getElements()) {
    if (doIt(elem.Type))
      return true;
  }
  return false;
}

bool Traversal::visitCompositionTypeRepr(CompositionTypeRepr *T) {
  for (auto elem : T->getTypes()) {
    if (doIt(elem))
      return true;
  }
  return false;
}

bool Traversal::visitMetatypeTypeRepr(MetatypeTypeRepr *T) {
  return doIt(T->getBase());
}

bool Traversal::visitProtocolTypeRepr(ProtocolTypeRepr *T) {
  return doIt(T->getBase());
}

bool Traversal::visitOwnershipTypeRepr(OwnershipTypeRepr *T) {
  return doIt(T->getBase());
}

bool Traversal::visitIsolatedTypeRepr(IsolatedTypeRepr *T) {
  return doIt(T->getBase());
}

bool Traversal::visitCompileTimeConstTypeRepr(CompileTimeConstTypeRepr *T) {
  return doIt(T->getBase());
}

bool Traversal::visitResultDependsOnTypeRepr(ResultDependsOnTypeRepr *T) {
  return doIt(T->getBase());
}

bool Traversal::visitOpaqueReturnTypeRepr(OpaqueReturnTypeRepr *T) {
  return doIt(T->getConstraint());
}

bool Traversal::visitNamedOpaqueReturnTypeRepr(NamedOpaqueReturnTypeRepr *T) {
  return doIt(T->getBase());
}

bool Traversal::visitExistentialTypeRepr(ExistentialTypeRepr *T) {
  return doIt(T->getConstraint());
}

bool Traversal::visitInverseTypeRepr(InverseTypeRepr *T) {
  return doIt(T->getConstraint());
}

bool Traversal::visitPlaceholderTypeRepr(PlaceholderTypeRepr *T) {
  return false;
}

bool Traversal::visitFixedTypeRepr(FixedTypeRepr *T) {
  return false;
}

bool Traversal::visitSelfTypeRepr(SelfTypeRepr *T) {
  return false;
}

bool Traversal::visitSILBoxTypeRepr(SILBoxTypeRepr *T) {
  for (auto &field : T->getFields()) {
    if (doIt(field.getFieldType()))
      return true;
  }
  for (auto &arg : T->getGenericArguments()) {
    if (doIt(arg))
      return true;
  }
  return false;
}

Expr *Expr::walk(ASTWalker &walker) {
  return Traversal(walker).doIt(this);
}

Stmt *Stmt::walk(ASTWalker &walker) {
  return Traversal(walker).doIt(this);
}

Pattern *Pattern::walk(ASTWalker &walker) {
  return Traversal(walker).doIt(this);
}

TypeRepr *TypeRepr::walk(ASTWalker &walker) {
  (void)Traversal(walker).doIt(this);
  return this;
}

StmtConditionElement *StmtConditionElement::walk(ASTWalker &walker) {
  (void)Traversal(walker).doIt(*this);
  return this;
}

bool Decl::walk(ASTWalker &walker) {
  return Traversal(walker).doIt(this);
}

bool GenericParamList::walk(ASTWalker &walker) {
  return Traversal(walker).doIt(this);
}

ArgumentList *ArgumentList::walk(ASTWalker &walker) {
  return Traversal(walker).doIt(this);
}