Revert r370850 "Re-commit r363191 "[MS] Pretend constexpr variable template specializations are inline""

This work-around was necessary to handle standard library headers in
Visual Studio 2019 16.2. Now that 16.3 has shipped to stable, we can
remove it.

> 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

llvm-svn: 372844

1 files changed, 4 insertions, 19 deletions

diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 96d0659d3e7..091b2fe3462 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -9915,25 +9915,10 @@ static GVALinkage basicGVALinkageForVariable(const ASTContext &Context,
     return StrongLinkage;
 
   case TSK_ExplicitSpecialization:
-    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;
+    return Context.getTargetInfo().getCXXABI().isMicrosoft() &&
+                   VD->isStaticDataMember()
+               ? GVA_StrongODR
+               : StrongLinkage;
 
   case TSK_ExplicitInstantiationDefinition:
     return GVA_StrongODR;