[ASTImporter] Fix infinite recurse on return type declared inside body (llvm#68775)

Lambda without trailing auto could have return type declared inside the
body too.

Fixes llvm#68775

Author: danix800

clang/docs/ReleaseNotes.rst

@@ -559,6 +559,9 @@ Bug Fixes to AST Handling
 Miscellaneous Bug Fixes
 ^^^^^^^^^^^^^^^^^^^^^^^
 
+- Fixed an infinite recursion in ASTImporter, on return type declared inside
+  body of C++11 lambda without trailing return (#GH68775).
+
 Miscellaneous Clang Crashes Fixed
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

clang/lib/AST/ASTImporter.cpp

@@ -695,7 +695,7 @@ namespace clang {
     // Returns true if the given function has a placeholder return type and
     // that type is declared inside the body of the function.
     // E.g. auto f() { struct X{}; return X(); }
-    bool hasAutoReturnTypeDeclaredInside(FunctionDecl *D);
+    bool hasReturnTypeDeclaredInside(FunctionDecl *D);
   };
 
 template <typename InContainerTy>
@@ -3647,15 +3647,28 @@ class IsTypeDeclaredInsideVisitor
 };
 } // namespace
 
-/// This function checks if the function has 'auto' return type that contains
+/// This function checks if the given function has a return type that contains
 /// a reference (in any way) to a declaration inside the same function.
-bool ASTNodeImporter::hasAutoReturnTypeDeclaredInside(FunctionDecl *D) {
+bool ASTNodeImporter::hasReturnTypeDeclaredInside(FunctionDecl *D) {
   QualType FromTy = D->getType();
   const auto *FromFPT = FromTy->getAs<FunctionProtoType>();
   assert(FromFPT && "Must be called on FunctionProtoType");
 
+  auto IsCXX11LambdaWithouTrailingReturn = [&]() {
+    if (Importer.FromContext.getLangOpts().CPlusPlus14) // C++14 or later
+      return false;
+
+    if (FromFPT->hasTrailingReturn())
+      return false;
+
+    if (const auto *MD = dyn_cast<CXXMethodDecl>(D))
+      return cast<CXXRecordDecl>(MD->getDeclContext())->isLambda();
+
+    return false;
+  };
+
   QualType RetT = FromFPT->getReturnType();
-  if (isa<AutoType>(RetT.getTypePtr())) {
+  if (isa<AutoType>(RetT.getTypePtr()) || IsCXX11LambdaWithouTrailingReturn()) {
     FunctionDecl *Def = D->getDefinition();
     IsTypeDeclaredInsideVisitor Visitor(Def ? Def : D);
     return Visitor.CheckType(RetT);
@@ -3811,7 +3824,7 @@ ExpectedDecl ASTNodeImporter::VisitFunctionDecl(FunctionDecl *D) {
     // E.g.: auto foo() { struct X{}; return X(); }
     // To avoid an infinite recursion when importing, create the FunctionDecl
     // with a simplified return type.
-    if (hasAutoReturnTypeDeclaredInside(D)) {
+    if (hasReturnTypeDeclaredInside(D)) {
       FromReturnTy = Importer.getFromContext().VoidTy;
       UsedDifferentProtoType = true;
     }

clang/unittests/AST/ASTImporterTest.cpp

@@ -6721,6 +6721,23 @@ TEST_P(ASTImporterOptionSpecificTestBase, LambdaInFunctionBody) {
   EXPECT_FALSE(FromL->isDependentLambda());
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase,
+       ReturnTypeDeclaredInsideOfCXX11LambdaWithoutTrailingReturn) {
+  Decl *From, *To;
+  std::tie(From, To) = getImportedDecl(
+      R"(
+      void foo() {
+        (void) []() {
+          struct X {};
+          return X();
+        };
+      }
+      )",
+      Lang_CXX11, "", Lang_CXX11, "foo"); // c++11 only
+  auto *ToLambda = FirstDeclMatcher<LambdaExpr>().match(To, lambdaExpr());
+  EXPECT_TRUE(ToLambda);
+}
+
 TEST_P(ASTImporterOptionSpecificTestBase, LambdaInFunctionParam) {
   Decl *FromTU = getTuDecl(
       R"(