[clang] [Clang] [Sema] Error on reference types inside a union with msvc 1910+ (PR #102851)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Max Winkler (MaxEW707)

<details>
<summary>Changes</summary>

Godbolt for reference: https://godbolt.org/z/ovKjvWc46

I definitely remember this being an extension in older versions of VS around VS 2012 but don't know when MSVC no longer exactly removed support for this extension wholesale.
I no longer have work licenses for VS2015 and VS2013.
I can confirm that this extension is no longer valid in VS2017, VS2019 and VS2022 under `/permissive` and `/permissive-`

I'll continue to spelunk MSDN and see if I can dig up one of old personal VS2015 licenses.

---
Full diff: https://github.com/llvm/llvm-project/pull/102851.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaDecl.cpp (+9-5) 
- (modified) clang/test/SemaCXX/MicrosoftExtensions.cpp (+16-5) 


``````````diff
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 694a754646f274..0386560e337618 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -18467,11 +18467,15 @@ FieldDecl *Sema::CheckFieldDecl(DeclarationName Name, QualType T,
       // the program is ill-formed, except when compiling with MSVC extensions
       // enabled.
       if (EltTy->isReferenceType()) {
-        Diag(NewFD->getLocation(), getLangOpts().MicrosoftExt ?
-                                    diag::ext_union_member_of_reference_type :
-                                    diag::err_union_member_of_reference_type)
-          << NewFD->getDeclName() << EltTy;
-        if (!getLangOpts().MicrosoftExt)
+        const bool HaveMSExt =
+            getLangOpts().MicrosoftExt &&
+            !getLangOpts().isCompatibleWithMSVC(LangOptions::MSVC2017);
+
+        Diag(NewFD->getLocation(),
+             HaveMSExt ? diag::ext_union_member_of_reference_type
+                       : diag::err_union_member_of_reference_type)
+            << NewFD->getDeclName() << EltTy;
+        if (!HaveMSExt)
           NewFD->setInvalidDecl();
       }
     }
diff --git a/clang/test/SemaCXX/MicrosoftExtensions.cpp b/clang/test/SemaCXX/MicrosoftExtensions.cpp
index 98c19975095bbe..8601f32f064b33 100644
--- a/clang/test/SemaCXX/MicrosoftExtensions.cpp
+++ b/clang/test/SemaCXX/MicrosoftExtensions.cpp
@@ -3,6 +3,8 @@
 // RUN: %clang_cc1 -std=c++11 %s -triple i686-pc-win32 -fsyntax-only -Wmicrosoft -Wc++11-extensions -Wno-long-long -verify=expected,precxx17 -fms-extensions -fexceptions -fcxx-exceptions -DTEST1
 // RUN: %clang_cc1 -std=c++14 %s -triple i686-pc-win32 -fsyntax-only -Wmicrosoft -Wc++11-extensions -Wno-long-long -verify=expected,precxx17 -fexceptions -fcxx-exceptions -DTEST2
 // RUN: %clang_cc1 %s -triple i686-pc-win32 -fsyntax-only -std=c++11 -fms-compatibility -verify -DTEST3
+// RUN: %clang_cc1 -std=c++17 %s -triple i686-pc-win32 -fsyntax-only -Wmicrosoft -verify -fms-extensions -fms-compatibility-version=19.00 -DTEST4
+// RUN: %clang_cc1 -std=c++17 %s -triple i686-pc-win32 -fsyntax-only -Wmicrosoft -verify -fms-extensions -fms-compatibility-version=19.10 -DTEST5
 
 #if TEST1
 
@@ -384,11 +386,6 @@ void TestSP9() {
   c3.h(); // Overloaded unary op operand
 }
 
-union u {
-  int *i1;
-  int &i2;  // expected-warning {{union member 'i2' has reference type 'int &', which is a Microsoft extension}}
-};
-
 // Property getter using reference.
 struct SP11 {
   __declspec(property(get=GetV)) int V;
@@ -619,6 +616,20 @@ template<typename T> struct A {};
 template<typename T> struct B : A<A<T>> { A<T>::C::D d; }; // expected-warning {{implicit 'typename' is a C++20 extension}}
 }
 
+#elif TEST4
+
+union u {
+    int *i1;
+    int &i2;  // expected-warning {{union member 'i2' has reference type 'int &', which is a Microsoft extension}}
+};
+
+#elif TEST5
+
+union u {
+    int *i1;
+    int &i2;  // expected-error {{union member 'i2' has reference type 'int &'}}
+};
+
 #else
 
 #error Unknown test mode

``````````

</details>


https://github.com/llvm/llvm-project/pull/102851