[AST] Fix double-traversal of code in top-level lambdas in RAV(implicit = yes).

[sammccall]

Summary

Prior to r351069, lambda classes were traversed or not depending on the
{Function, Class, Namespace, TU} DeclContext containing them.
If it was a function (common case) they were not traversed.
If it was a namespace or TU (top-level lambda) they were traversed as part of
that DeclContext traversal.

r351069 "fixed" RAV to traverse these as part of the LambdaExpr, which is the
right place. But top-level lambdas are now traversed twice.
We fix that as blocks and block captures were apparently fixed in the past.

Maybe it would be nicer to avoid adding the lambda classes to the DeclContext
in the first place, but I can't work out the implications of that.

[sammccall]

unittests/Tooling/RecursiveASTVisitorTests/LambdaExpr.cpp

72

Dirty secret: this is the line that catches the old bug.
The outer lambda was only traversed once (because the *lambdaexpr* was only traversed once) but its body, and thus the inner lambda was traversed twice (causing an assertion).

It'd be possible to express more directly, but it would require hacking up the fixture more, and this new test conveniently tests the lambda-in-lambda-with-VisitImplicitCode case that I forgot in the last patch.

[bkramer]

This makes sense to me.

include/clang/AST/RecursiveASTVisitor.h

 // For a Decl, we automate (in the DEF_TRAVERSE_DECL macro) traversing
 // the children that come from the DeclContext associated with it.
 // Therefore each Traverse* only needs to worry about children other
 // than those.
 
 template <typename Derived>
 bool RecursiveASTVisitor<Derived>::canIgnoreChildDeclWhileTraversingDeclContext(
     const Decl *Child) {
-  // BlockDecls and CapturedDecls are traversed through BlockExprs and
-  // CapturedStmts respectively.
-  return isa<BlockDecl>(Child) || isa<CapturedDecl>(Child);
+  // BlockDecls are traversed through BlockExprs,
+  // CapturedDecls are traversed through CapturedStmts.
+  if (isa<BlockDecl>(Child) || isa<CapturedDecl>(Child))
+    return true;
+  // Lambda classes are traversed through LambdaExprs.
+  if (const CXXRecordDecl* Cls = dyn_cast<CXXRecordDecl>(Child))
+    return Cls->isLambda();
+  return false;
 }
 
 template <typename Derived>
 bool RecursiveASTVisitor<Derived>::TraverseDeclContextHelper(DeclContext *DC) {
   if (!DC)
     return true;
 
   for (auto *Child : DC->decls()) {

unittests/Tooling/RecursiveASTVisitorTests/LambdaExpr.cpp

TEST(RecursiveASTVisitor, LambdaInLambda) {
   Visitor.ExpectMatch("", 1, 12);
   Visitor.ExpectMatch("", 1, 16);
   EXPECT_TRUE(Visitor.runOver("void f() { []{ []{ return; }; }(); }",
                               LambdaExprVisitor::Lang_CXX11));
   EXPECT_TRUE(Visitor.allBodiesHaveBeenTraversed());
   EXPECT_FALSE(Visitor.allClassesHaveBeenTraversed());
 }
 
+TEST(RecursiveASTVisitor, TopLevelLambda) {
+  LambdaExprVisitor Visitor;
+  Visitor.VisitImplicitCode = true;
+  Visitor.ExpectMatch("", 1, 10);
+  Visitor.ExpectMatch("", 1, 14);
+  EXPECT_TRUE(Visitor.runOver("auto x = []{ [] {}; };",
+                              LambdaExprVisitor::Lang_CXX11));
+  EXPECT_TRUE(Visitor.allBodiesHaveBeenTraversed());
+  EXPECT_TRUE(Visitor.allClassesHaveBeenTraversed());
+}
+
 TEST(RecursiveASTVisitor, VisitsLambdaExprAndImplicitClass) {
   LambdaExprVisitor Visitor;
   Visitor.VisitImplicitCode = true;
   Visitor.ExpectMatch("", 1, 12);
   EXPECT_TRUE(Visitor.runOver("void f() { []{ return; }(); }",
                               LambdaExprVisitor::Lang_CXX11));
   EXPECT_TRUE(Visitor.allBodiesHaveBeenTraversed());
   EXPECT_TRUE(Visitor.allClassesHaveBeenTraversed());