[MS] Don't expect vftables to be provided for extern template instantiations

MSVC doesn't provide them. PR28223

I left behind the machinery in case we want to resurrect available_externally
vftable emission to support devirtualization.

Reviewers: majnemer

Differential Revision: http://reviews.llvm.org/D21544

llvm-svn: 273296

1 files changed, 6 insertions, 7 deletions

diff --git a/clang/lib/AST/VTableBuilder.cpp b/clang/lib/AST/VTableBuilder.cpp
index e43acc4de78..640fbf47aea 100644
--- a/clang/lib/AST/VTableBuilder.cpp
+++ b/clang/lib/AST/VTableBuilder.cpp
@@ -2545,14 +2545,13 @@ public:
         MostDerivedClassLayout(Context.getASTRecordLayout(MostDerivedClass)),
         WhichVFPtr(*Which),
         Overriders(MostDerivedClass, CharUnits(), MostDerivedClass) {
-    // Only include the RTTI component if we know that we will provide a
-    // definition of the vftable.  We always provide the definition of
-    // dllimported classes.
+    // Provide the RTTI component if RTTIData is enabled. If the vftable would
+    // be available externally, we should not provide the RTTI componenent. It
+    // is currently impossible to get available externally vftables with either
+    // dllimport or extern template instantiations, but eventually we may add a
+    // flag to support additional devirtualization that needs this.
     if (Context.getLangOpts().RTTIData)
-      if (MostDerivedClass->hasAttr<DLLImportAttr>() ||
-          MostDerivedClass->getTemplateSpecializationKind() !=
-              TSK_ExplicitInstantiationDeclaration)
-        HasRTTIComponent = true;
+      HasRTTIComponent = true;
 
     LayoutVFTable();