[MS] Pretend constexpr variable template specializations are inline

[rnk]

Summary

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.

[rsmith]

clang/lib/AST/ASTContext.cpp

9809

It'd be nice to include a note here that this is strictly non-conforming (since another TU could be relying on this TU to provide the definition), but that we don't expect that to happen in practice for variable template specializations declared constexpr.

[rnk]

clang/lib/AST/ASTContext.cpp

9809

I forgot to add the comment before pushing, so I added it in rC363195.

clang/lib/AST/ASTContext.cpp

case ASTContext::InlineVariableDefinitionKind::Strong:
     break;
   }
 
   switch (VD->getTemplateSpecializationKind()) {
   case TSK_Undeclared:
     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 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;
 
   case TSK_ExplicitInstantiationDeclaration:
     return GVA_AvailableExternally;
 
   case TSK_ImplicitInstantiation:

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

+// 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