[clang] [clang][ASTImporter] delay import funtion return type (PR #74991)

[Qizhi Hu via cfe-commits]

https://github.com/jcsxky created https://github.com/llvm/llvm-project/pull/74991

Import return type of a function would lead infinite recursion when `getAssociatedDecl()` returns itself in `ASTNodeImporter::VisitSubstNonTypeTemplateParmExpr`. Delay import the return type whether it is auto would make sense. This patch try to fix [this issue](https://github.com/llvm/llvm-project/issues/74839)

>From 1527cc2046d4c9881ca536a2dbf59330d9291ba2 Mon Sep 17 00:00:00 2001
From: huqizhi <huqizhi at feysh.com>
Date: Sun, 10 Dec 2023 21:01:49 +0800
Subject: [PATCH] [clang][ASTImporter] delay import funtion return type

---
 clang/lib/AST/ASTImporter.cpp           |  8 ++------
 clang/unittests/AST/ASTImporterTest.cpp | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index f1f335118f37a4..335d10016f2060 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -3739,16 +3739,13 @@ ExpectedDecl ASTNodeImporter::VisitFunctionDecl(FunctionDecl *D) {
   // do the same with TypeSourceInfo.
   bool UsedDifferentProtoType = false;
   if (const auto *FromFPT = FromTy->getAs<FunctionProtoType>()) {
-    QualType FromReturnTy = FromFPT->getReturnType();
     // Functions with auto return type may define a struct inside their body
     // and the return type could refer to that struct.
     // 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)) {
-      FromReturnTy = Importer.getFromContext().VoidTy;
-      UsedDifferentProtoType = true;
-    }
+    QualType FromReturnTy = Importer.getFromContext().VoidTy;
+    UsedDifferentProtoType = true;
     FunctionProtoType::ExtProtoInfo FromEPI = FromFPT->getExtProtoInfo();
     // FunctionProtoType::ExtProtoInfo's ExceptionSpecDecl can point to the
     // FunctionDecl that we are importing the FunctionProtoType for.
@@ -3759,7 +3756,6 @@ ExpectedDecl ASTNodeImporter::VisitFunctionDecl(FunctionDecl *D) {
         FromEPI.ExceptionSpec.NoexceptExpr) {
       FunctionProtoType::ExtProtoInfo DefaultEPI;
       FromEPI = DefaultEPI;
-      UsedDifferentProtoType = true;
     }
     FromTy = Importer.getFromContext().getFunctionType(
         FromReturnTy, FromFPT->getParamTypes(), FromEPI);
diff --git a/clang/unittests/AST/ASTImporterTest.cpp b/clang/unittests/AST/ASTImporterTest.cpp
index 4dd7510bf8ddf8..7ab9db4251e80b 100644
--- a/clang/unittests/AST/ASTImporterTest.cpp
+++ b/clang/unittests/AST/ASTImporterTest.cpp
@@ -9284,6 +9284,26 @@ TEST_P(ASTImporterOptionSpecificTestBase,
   // EXPECT_EQ(ToF1Imported->getPreviousDecl(), ToF1);
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase, ImportFunctionAutoType) {
+  const char *Code =
+      R"(
+      template<int>
+      struct array {};
+
+      template <int N>
+      auto foo() { return array<N>(); }
+
+      void bar() { foo<0>(); }
+      )";
+  Decl *FromTU = getTuDecl(Code, Lang_CXX17);
+
+  auto *FromBar = FirstDeclMatcher<FunctionDecl>().match(
+      FromTU, functionDecl(hasName("bar")));
+
+  auto *ToBar = Import(FromBar, Lang_CXX17);
+  EXPECT_TRUE(ToBar);
+}
+
 INSTANTIATE_TEST_SUITE_P(ParameterizedTests, ASTImporterLookupTableTest,
                          DefaultTestValuesForRunOptions);