[MS] Silence -Wextern-init on const selectany variables

In C, 'extern' is typically used to avoid tentative definitions when
declaring variables in headers, but adding an intializer makes it a
defintion. This is somewhat confusing, so GCC and Clang both warn on it.
In C++, 'extern' is often used to give implictly static 'const'
variables external linkage, so don't warn in that case. If selectany is
present, this might be header code intended for C and C++ inclusion, so
apply the C++ rules.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@279116 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Sema/SemaDecl.cpp

@@ -9938,10 +9938,17 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init,
       VDecl->setInvalidDecl();
     }
   } else if (VDecl->isFileVarDecl()) {
+    // In C, extern is typically used to avoid tentative definitions when
+    // declaring variables in headers, but adding an intializer makes it a
+    // defintion. This is somewhat confusing, so GCC and Clang both warn on it.
+    // In C++, extern is often used to give implictly static const variables
+    // external linkage, so don't warn in that case. If selectany is present,
+    // this might be header code intended for C and C++ inclusion, so apply the
+    // C++ rules.
     if (VDecl->getStorageClass() == SC_Extern &&
-        (!getLangOpts().CPlusPlus ||
-         !(Context.getBaseElementType(VDecl->getType()).isConstQualified() ||
-           VDecl->isExternC())) &&
+        ((!getLangOpts().CPlusPlus && !VDecl->hasAttr<SelectAnyAttr>()) ||
+         !Context.getBaseElementType(VDecl->getType()).isConstQualified()) &&
+        !(getLangOpts().CPlusPlus && VDecl->isExternC()) &&
         !isTemplateInstantiation(VDecl->getTemplateSpecializationKind()))
       Diag(VDecl->getLocation(), diag::warn_extern_init);

test/Sema/attr-selectany.c

+// RUN: %clang_cc1 -fms-compatibility -fms-extensions -verify %s
+
+extern __declspec(selectany) const int x1 = 1; // no warning, const means we need extern in C++
+
+// Should we really warn on this?
+extern __declspec(selectany) int x2 = 1; // expected-warning {{'extern' variable has an initializer}}
+
+__declspec(selectany) void foo() { } // expected-error{{'selectany' can only be applied to data items with external linkage}}