When "delayed parsing" C++ default arguments, if there is an error, there may be tokens left in the token stream

that will interfere (they will be parsed as if they are after the class' '}') and a crash will occur because
the CachedTokens that holds them will be deleted while the lexer is still using them.

Make sure that the tokens of default args are removed from the token stream.
Fixes PR6647.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@99939 91177308-0d34-0410-b5e6-96231b3b80d8

Author: akyrtzi

lib/Parse/ParseCXXInlineMethods.cpp

@@ -122,6 +122,9 @@ void Parser::ParseLexedMethodDeclarations(ParsingClass &Class) {
       Actions.ActOnDelayedCXXMethodParameter(CurScope, LM.DefaultArgs[I].Param);
 
       if (CachedTokens *Toks = LM.DefaultArgs[I].Toks) {
+        // Save the current token position.
+        SourceLocation origLoc = Tok.getLocation();
+
         // Parse the default argument from its saved token stream.
         Toks->push_back(Tok); // So that the current token doesn't get lost
         PP.EnterTokenStream(&Toks->front(), Toks->size(), true, false);
@@ -139,6 +142,15 @@ void Parser::ParseLexedMethodDeclarations(ParsingClass &Class) {
         else
           Actions.ActOnParamDefaultArgument(LM.DefaultArgs[I].Param, EqualLoc,
                                             move(DefArgResult));
+
+        assert(!PP.getSourceManager().isBeforeInTranslationUnit(origLoc,
+                                                           Tok.getLocation()) &&
+               "ParseAssignmentExpression went over the default arg tokens!");
+        // There could be leftover tokens (e.g. because of an error).
+        // Skip through until we reach the original token position.
+        while (Tok.getLocation() != origLoc)
+          ConsumeAnyToken();
+
         delete Toks;
         LM.DefaultArgs[I].Toks = 0;
       }

test/Parser/cxx-default-args.cpp

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+// PR6647
+class C {
+  // After the error, the rest of the tokens inside the default arg should be
+  // skipped, avoiding a "expected ';' after class" after 'undecl'.
+  void m(int x = undecl + 0); // expected-error {{use of undeclared identifier 'undecl'}}
+};
+