[clang][ASTImporter] Remove trailing return testing on lambda proto (#101031)

Lambdas without trailing return could also have return type defined
inside its body.

This fixes crashes (infinite recursion) on lambda expr without
parameters (no parentheses).

Author: danix800

clang/lib/AST/ASTImporter.cpp

@@ -3717,21 +3717,18 @@ bool ASTNodeImporter::hasReturnTypeDeclaredInside(FunctionDecl *D) {
   const auto *FromFPT = FromTy->getAs<FunctionProtoType>();
   assert(FromFPT && "Must be called on FunctionProtoType");
 
-  auto IsCXX11LambdaWithouTrailingReturn = [&]() {
+  auto IsCXX11Lambda = [&]() {
     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()) || IsCXX11LambdaWithouTrailingReturn()) {
+  if (isa<AutoType>(RetT.getTypePtr()) || IsCXX11Lambda()) {
     FunctionDecl *Def = D->getDefinition();
     IsTypeDeclaredInsideVisitor Visitor(Def ? Def : D);
     return Visitor.CheckType(RetT);

clang/unittests/AST/ASTImporterTest.cpp

@@ -6738,6 +6738,23 @@ TEST_P(ASTImporterOptionSpecificTestBase,
   EXPECT_TRUE(ToLambda);
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase,
+       ReturnTypeDeclaredInsideOfCXX11LambdaWithTrailingReturn) {
+  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"(