[clang] [clang][ASTImporter] Fix import of SubstTemplateTypeParmType in return type of function. (PR #69724)

[Balázs Kéri via cfe-commits]

https://github.com/balazske created https://github.com/llvm/llvm-project/pull/69724

Import of a function with `auto` return type that is expanded to a `SubstTemplateTypeParmType` could fail if the function itself is the template specialization where the parameter was replaced.

>From 2bfad6f0fbb7a5757318c77ef76335986d23ab83 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bal=C3=A1zs=20K=C3=A9ri?= <balazs.keri at ericsson.com>
Date: Thu, 19 Oct 2023 16:27:24 +0200
Subject: [PATCH] [clang][ASTImporter] Fix import of SubstTemplateTypeParmType
 in return type of function.

Import of a function with `auto` return type that is expanded to a
`SubstTemplateTypeParmType` could fail if the function itself is the template
specialization where the parameter was replaced.
---
 clang/lib/AST/ASTImporter.cpp           | 10 ++++++++++
 clang/unittests/AST/ASTImporterTest.cpp | 23 ++++++++++++++++++++---
 2 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index 628a2b2bbca3986..dbcd4e616ecf03c 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -3513,6 +3513,14 @@ class IsTypeDeclaredInsideVisitor
     return {};
   }
 
+  std::optional<bool>
+  VisitSubstTemplateTypeParmType(const SubstTemplateTypeParmType *T) {
+    // The "associated declaration" can be the same as ParentDC.
+    if (isAncestorDeclContextOf(ParentDC, T->getAssociatedDecl()))
+      return true;
+    return {};
+  }
+
   std::optional<bool> VisitConstantArrayType(const ConstantArrayType *T) {
     if (T->getSizeExpr() && isAncestorDeclContextOf(ParentDC, T->getSizeExpr()))
       return true;
@@ -3573,6 +3581,8 @@ class IsTypeDeclaredInsideVisitor
 };
 } // namespace
 
+/// This function checks if the function has 'auto' return type that contains
+/// a reference (in any way) to a declaration inside the same function.
 bool ASTNodeImporter::hasAutoReturnTypeDeclaredInside(FunctionDecl *D) {
   QualType FromTy = D->getType();
   const auto *FromFPT = FromTy->getAs<FunctionProtoType>();
diff --git a/clang/unittests/AST/ASTImporterTest.cpp b/clang/unittests/AST/ASTImporterTest.cpp
index f1f09a0be2b8d0c..21c24c2fcdc0007 100644
--- a/clang/unittests/AST/ASTImporterTest.cpp
+++ b/clang/unittests/AST/ASTImporterTest.cpp
@@ -6766,10 +6766,13 @@ TEST_P(ASTImporterOptionSpecificTestBase,
 }
 
 struct ImportAutoFunctions : ASTImporterOptionSpecificTestBase {
-  void testImport(llvm::StringRef Code, clang::TestLanguage Lang = Lang_CXX14) {
+  void testImport(llvm::StringRef Code, clang::TestLanguage Lang = Lang_CXX14,
+                  bool FindLast = false) {
     Decl *FromTU = getTuDecl(Code, Lang, "input0.cc");
-    FunctionDecl *From = FirstDeclMatcher<FunctionDecl>().match(
-        FromTU, functionDecl(hasName("foo")));
+    FunctionDecl *From = FindLast ? LastDeclMatcher<FunctionDecl>().match(
+                                        FromTU, functionDecl(hasName("foo")))
+                                  : FirstDeclMatcher<FunctionDecl>().match(
+                                        FromTU, functionDecl(hasName("foo")));
 
     FunctionDecl *To = Import(From, Lang);
     EXPECT_TRUE(To);
@@ -7208,6 +7211,20 @@ TEST_P(ImportAutoFunctions, ReturnWithTypeInSwitch) {
       Lang_CXX17);
 }
 
+TEST_P(ImportAutoFunctions, ReturnWithAutoTemplateType) {
+  testImport(
+      R"(
+      template<class T>
+      struct S {};
+      template<class T>
+      auto foo() {
+        return S<T>{};
+      }
+      auto a = foo<int>(1);
+      )",
+      Lang_CXX14, /*FindLast=*/true);
+}
+
 struct ImportSourceLocations : ASTImporterOptionSpecificTestBase {};
 
 TEST_P(ImportSourceLocations, PreserveFileIDTreeStructure) {