From 7b3fe969927731c69ba4d8a428442e1e191f49b5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Storsj=C3=B6?= <martin@martin.st>
Date: Sat, 13 Jun 2020 22:29:52 +0300
Subject: [PATCH] [clang] Don't emit warn_cxx_ms_struct when MSBitfields is
 enabled globally

This diagnostic (which defaults to an error, added in
95833f33bda6c92e746e0b0007b69c2c30bfc693) was intended to clearly
point out cases where the C++ ABI won't match the Microsoft C++ ABI,
for cases when this is enabled via a pragma over a region of code.

The MSVC compatible struct layout feature can also be enabled via a
compiler option (-mms-bitfields). If enabled that way, one essentially
can't compile any C++ code unless also building with
-Wno-incompatible-ms-struct (which GCC doesn't support, and projects
developed with GCC aren't setting).

For the MinGW target, it's expected that the C++ ABI won't match
the MSVC one, if this option is used for getting the struct
layout to match MSVC.

Differential Revision: https://reviews.llvm.org/D81794
---
 clang/lib/Sema/SemaDeclCXX.cpp   |  6 +++++-
 clang/test/SemaCXX/ms_struct.cpp | 14 ++++++++++----
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index f1ade752e2fe..a2b669c99125 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -6773,7 +6773,11 @@ void Sema::CheckCompletedCXXClass(Scope *S, CXXRecordDecl *Record) {
   // headers, sweeping up a bunch of types that the project doesn't
   // really rely on MSVC-compatible layout for.  We must therefore
   // support "ms_struct except for C++ stuff" as a secondary ABI.
-  if (Record->isMsStruct(Context) &&
+  // Don't emit this diagnostic if the feature was enabled as a
+  // language option (as opposed to via a pragma or attribute), as
+  // the option -mms-bitfields otherwise essentially makes it impossible
+  // to build C++ code, unless this diagnostic is turned off.
+  if (Record->isMsStruct(Context) && !Context.getLangOpts().MSBitfields &&
       (Record->isPolymorphic() || Record->getNumBases())) {
     Diag(Record->getLocation(), diag::warn_cxx_ms_struct);
   }
diff --git a/clang/test/SemaCXX/ms_struct.cpp b/clang/test/SemaCXX/ms_struct.cpp
index 414b56b491c6..122819c3eead 100644
--- a/clang/test/SemaCXX/ms_struct.cpp
+++ b/clang/test/SemaCXX/ms_struct.cpp
@@ -1,8 +1,11 @@
-// RUN: %clang_cc1 -fsyntax-only -Wno-error=incompatible-ms-struct -verify -triple i686-apple-darwin9 -std=c++11 %s
-// RUN: %clang_cc1 -fsyntax-only -Wno-error=incompatible-ms-struct -verify -triple armv7-apple-darwin9 -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -DTEST_FOR_WARNING -Wno-error=incompatible-ms-struct -verify -triple i686-apple-darwin9 -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -DTEST_FOR_WARNING -Wno-error=incompatible-ms-struct -verify -triple armv7-apple-darwin9 -std=c++11 %s
 // RUN: %clang_cc1 -fsyntax-only -DTEST_FOR_ERROR -verify -triple armv7-apple-darwin9 -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -DNO_PRAGMA -mms-bitfields -verify -triple armv7-apple-darwin9 -std=c++11 %s
 
+#ifndef NO_PRAGMA
 #pragma ms_struct on
+#endif
 
 struct A {
   unsigned long a:4;
@@ -12,7 +15,7 @@ struct A {
 struct B : public A {
 #ifdef TEST_FOR_ERROR
   // expected-error@-2 {{ms_struct may not produce Microsoft-compatible layouts for classes with base classes or virtual functions}}
-#else
+#elif defined(TEST_FOR_WARNING)
   // expected-warning@-4 {{ms_struct may not produce Microsoft-compatible layouts for classes with base classes or virtual functions}}
 #endif
   unsigned long c:16;
@@ -27,7 +30,7 @@ static_assert(__builtin_offsetof(B, d) == 12,
 struct C {
 #ifdef TEST_FOR_ERROR
   // expected-error@-2 {{ms_struct may not produce Microsoft-compatible layouts for classes with base classes or virtual functions}}
-#else
+#elif defined(TEST_FOR_WARNING)
   // expected-warning@-4 {{ms_struct may not produce Microsoft-compatible layouts for classes with base classes or virtual functions}}
 #endif
   virtual void foo();
@@ -36,3 +39,6 @@ struct C {
 
 static_assert(__builtin_offsetof(C, n) == 8,
               "long long field in ms_struct should be 8-byte aligned");
+#if !defined(TEST_FOR_ERROR) && !defined(TEST_FOR_WARNING)
+// expected-no-diagnostics
+#endif