From c55f729746794e33c789e49fac7c20b146f4fce1 Mon Sep 17 00:00:00 2001
From: Alexey Bataev <a.bataev@hotmail.com>
Date: Wed, 4 Feb 2015 04:45:32 +0000
Subject: [PATCH] [MSVC2012] Allow 'mutable' references Some standard header
 files from MSVC2012 use 'mutable' on references, though it is directly
 prohibited by the standard. Fix for http://llvm.org/PR22444 Differential
 Revision: http://reviews.llvm.org/D7370

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@228113 91177308-0d34-0410-b5e6-96231b3b80d8
---
 include/clang/Basic/DiagnosticSemaKinds.td   |  3 +++
 lib/Sema/SemaDecl.cpp                        |  9 ++++++---
 test/SemaCXX/ms_mutable_reference_member.cpp | 13 +++++++++++++
 3 files changed, 22 insertions(+), 3 deletions(-)
 create mode 100644 test/SemaCXX/ms_mutable_reference_member.cpp

diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index b17ebf6cf8..0ca9edfab7 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/include/clang/Basic/DiagnosticSemaKinds.td
@@ -1241,6 +1241,9 @@ def err_storageclass_invalid_for_member : Error<
   "storage class specified for a member declaration">;
 def err_mutable_function : Error<"'mutable' cannot be applied to functions">;
 def err_mutable_reference : Error<"'mutable' cannot be applied to references">;
+def ext_mutable_reference : ExtWarn<
+  "'mutable' on a reference type is a Microsoft extension">,
+  InGroup<Microsoft>;
 def err_mutable_const : Error<"'mutable' and 'const' cannot be mixed">;
 def err_mutable_nonmember : Error<
   "'mutable' can only be applied to member variables">;
diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index eb21dff766..a5ce9a0277 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -12348,7 +12348,8 @@ FieldDecl *Sema::CheckFieldDecl(DeclarationName Name, QualType T,
   if (!InvalidDecl && Mutable) {
     unsigned DiagID = 0;
     if (T->isReferenceType())
-      DiagID = diag::err_mutable_reference;
+      DiagID = getLangOpts().MSVCCompat ? diag::ext_mutable_reference
+                                        : diag::err_mutable_reference;
     else if (T.isConstQualified())
       DiagID = diag::err_mutable_const;
 
@@ -12357,8 +12358,10 @@ FieldDecl *Sema::CheckFieldDecl(DeclarationName Name, QualType T,
       if (D && D->getDeclSpec().getStorageClassSpecLoc().isValid())
         ErrLoc = D->getDeclSpec().getStorageClassSpecLoc();
       Diag(ErrLoc, DiagID);
-      Mutable = false;
-      InvalidDecl = true;
+      if (DiagID != diag::ext_mutable_reference) {
+        Mutable = false;
+        InvalidDecl = true;
+      }
     }
   }
 
diff --git a/test/SemaCXX/ms_mutable_reference_member.cpp b/test/SemaCXX/ms_mutable_reference_member.cpp
new file mode 100644
index 0000000000..ae1389d333
--- /dev/null
+++ b/test/SemaCXX/ms_mutable_reference_member.cpp
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 %s -fsyntax-only -verify -fms-compatibility
+
+struct S {
+  mutable int &a; // expected-warning {{'mutable' on a reference type is a Microsoft extension}}
+  S(int &b) : a(b) {}
+};
+
+int main() {
+  int a = 0;
+  const S s(a);
+  s.a = 10;
+  return s.a + a;
+}