From: Hans Wennborg <hans@hanshq.net>
Date: Wed, 25 Sep 2019 11:09:46 +0000 (+0000)
Subject: Revert r370850 "Re-commit r363191 "[MS] Pretend constexpr variable template specializ... 
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=c213673642e93493395fe4b28c8dd4e876f13995;p=clang

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@372844 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/AST/ASTContext.cpp b/lib/AST/ASTContext.cpp
index 96d0659d3e..091b2fe346 100644
--- a/lib/AST/ASTContext.cpp
+++ b/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;
diff --git a/test/CodeGenCXX/ms-constexpr-var-template.cpp b/test/CodeGenCXX/ms-constexpr-var-template.cpp
deleted file mode 100644
index a40f7aacac..0000000000
--- a/test/CodeGenCXX/ms-constexpr-var-template.cpp
+++ /dev/null
@@ -1,11 +0,0 @@
-// 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