[Clang] [Sema] Error on reference types inside a union with msvc 1900+ (#102851)

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

I can confirm that this extension is no longer valid in VS2017, VS2019
and VS2022 under `/permissive` and `/permissive-`

MSDN, https://learn.microsoft.com/en-us/cpp/porting/visual-cpp-what-s-new-2003-through-2015?view=msvc-170, says this extension was officially removed in VS2015.

Author: MaxEW707

clang/docs/ReleaseNotes.rst

@@ -321,6 +321,12 @@ Android Support
 Windows Support
 ^^^^^^^^^^^^^^^
 
+- Clang no longer allows references inside a union when emulating MSVC 1900+ even if `fms-extensions` is enabled.
+  Starting with VS2015, MSVC 1900, this Microsoft extension is no longer allowed and always results in an error.
+  Clang now follows the MSVC behavior in this scenario.
+  When `-fms-compatibility-version=18.00` or prior is set on the command line this Microsoft extension is still
+  allowed as VS2013 and prior allow it.
+
 LoongArch Support
 ^^^^^^^^^^^^^^^^^
 

clang/lib/Sema/SemaDecl.cpp

@@ -18507,11 +18507,15 @@ 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::MSVC2015);
+
+        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();
       }
     }

clang/test/SemaCXX/MicrosoftExtensions.cpp

@@ -1,8 +1,10 @@
-// RUN: %clang_cc1 -std=c++17 %s -triple i686-pc-win32 -fsyntax-only -Wmicrosoft -Wc++11-extensions -Wno-long-long -verify -fms-extensions -fexceptions -fcxx-exceptions -DTEST1
-// RUN: %clang_cc1 -std=c++98 %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++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 -Wc++11-extensions -Wno-long-long -verify=expected,ms-union-ext -fms-extensions -fexceptions -fcxx-exceptions -DTEST1
+// RUN: %clang_cc1 -std=c++98 %s -triple i686-pc-win32 -fsyntax-only -Wmicrosoft -Wc++11-extensions -Wno-long-long -verify=expected,precxx17,ms-union-ext -fms-extensions -fexceptions -fcxx-exceptions -DTEST1
+// RUN: %clang_cc1 -std=c++11 %s -triple i686-pc-win32 -fsyntax-only -Wmicrosoft -Wc++11-extensions -Wno-long-long -verify=expected,precxx17,ms-union-ext -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,ms-union-ext-disabled -fexceptions -fcxx-exceptions -DTEST2
+// RUN: %clang_cc1 %s -triple i686-pc-win32 -fsyntax-only -std=c++11 -fms-compatibility -verify=expected,ms-union-ext -DTEST3
+// RUN: %clang_cc1 -std=c++17 %s -triple i686-pc-win32 -fsyntax-only -Wmicrosoft -verify=ms-union-ext -fms-extensions -fms-compatibility-version=18.00
+// RUN: %clang_cc1 -std=c++17 %s -triple i686-pc-win32 -fsyntax-only -Wmicrosoft -verify=ms-union-ext-disabled -fms-extensions -fms-compatibility-version=19.00
 
 #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;
@@ -619,9 +616,12 @@ 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}}
 }
 
-#else
-
-#error Unknown test mode
-
 #endif
 
+union u {
+    int *i1;
+
+    // ms-union-ext-warning@+2 {{union member 'i2' has reference type 'int &', which is a Microsoft extension}}
+    // ms-union-ext-disabled-error@+1 {{union member 'i2' has reference type 'int &'}}
+    int &i2;
+};