Re-commit r363191 "[MS] Pretend constexpr variable template specializations are inline"

While the next Visual Studio update (16.3) will fix this issue, that hasn't
shipped yet. Until then Clang wouldn't work with MSVC's headers which seems
unfortunate. Let's keep this in until VS 16.3 ships. (See also PR42843.)

> Fixes link errors with clang and the latest Visual C++ 14.21.27702
> headers, which was reported as PR42027.
>
> I chose to intentionally make these things linkonce_odr, i.e.
> discardable, so that we don't emit definitions of these things in every
> translation unit that includes STL headers.
>
> Note that this is *not* what MSVC does: MSVC has not yet implemented C++
> DR2387, so they emit fully specialized constexpr variable templates with
> static / internal linkage.
>
> Reviewers: rsmith
>
> Differential Revision: https://reviews.llvm.org/D63175

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@370850 91177308-0d34-0410-b5e6-96231b3b80d8

lib/AST/ASTContext.cpp

@@ -9905,10 +9905,25 @@ static GVALinkage basicGVALinkageForVariable(const ASTContext &Context,
     return StrongLinkage;
 
   case TSK_ExplicitSpecialization:
-    return Context.getTargetInfo().getCXXABI().isMicrosoft() &&
-                   VD->isStaticDataMember()
-               ? GVA_StrongODR
-               : StrongLinkage;
+    if (Context.getTargetInfo().getCXXABI().isMicrosoft()) {
+      // If this is a fully specialized constexpr variable template, pretend it
+      // was marked inline. MSVC 14.21.27702 headers define _Is_integral in a
+      // header this way, and we don't want to emit non-discardable definitions
+      // of these variables in every TU that includes <type_traits>. This
+      // behavior is non-conforming, since another TU could use an extern
+      // template declaration for this variable, but for constexpr variables,
+      // it's unlikely for a user to want to do that. This behavior can be
+      // removed if the headers change to explicitly mark such variable template
+      // specializations inline.
+      if (isa<VarTemplateSpecializationDecl>(VD) && VD->isConstexpr())
+        return GVA_DiscardableODR;
+
+      // Use ODR linkage for static data members of fully specialized templates
+      // to prevent duplicate definition errors with MSVC.
+      if (VD->isStaticDataMember())
+        return GVA_StrongODR;
+    }
+    return StrongLinkage;
 
   case TSK_ExplicitInstantiationDefinition:
     return GVA_StrongODR;

test/CodeGenCXX/ms-constexpr-var-template.cpp

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -emit-llvm -triple=x86_64-windows-msvc -fms-compatibility %s -o - | FileCheck %s
+
+template <typename> constexpr bool _Is_integer = false;
+template <> constexpr bool _Is_integer<int> = true;
+template <> constexpr bool _Is_integer<char> = false;
+extern "C" const bool *escape = &_Is_integer<int>;
+
+// CHECK: @"??$_Is_integer@H@@3_NB" = linkonce_odr dso_local constant i8 1, comdat, align 1
+//   Should not emit _Is_integer<char>, since it's not referenced.
+// CHECK-NOT: @"??$_Is_integer@D@@3_NB"
+// CHECK: @escape = dso_local global i8* @"??$_Is_integer@H@@3_NB", align 8