2 files changed, 21 insertions, 14 deletions

diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 6ccb3c5216b..ed77a42beba 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -14281,6 +14281,8 @@ bool Sema::DefineUsedVTables() {
     CXXRecordDecl *Class = VTableUses[I].first->getDefinition();
     if (!Class)
       continue;
+    TemplateSpecializationKind ClassTSK =
+        Class->getTemplateSpecializationKind();
 
     SourceLocation Loc = VTableUses[I].second;
 
@@ -14304,9 +14306,8 @@ bool Sema::DefineUsedVTables() {
       // of an explicit instantiation declaration, suppress the
       // vtable; it will live with the explicit instantiation
       // definition.
-      bool IsExplicitInstantiationDeclaration
-        = Class->getTemplateSpecializationKind()
-                                      == TSK_ExplicitInstantiationDeclaration;
+      bool IsExplicitInstantiationDeclaration =
+          ClassTSK == TSK_ExplicitInstantiationDeclaration;
       for (auto R : Class->redecls()) {
         TemplateSpecializationKind TSK
           = cast<CXXRecordDecl>(R)->getTemplateSpecializationKind();
@@ -14339,17 +14340,20 @@ bool Sema::DefineUsedVTables() {
     if (VTablesUsed[Canonical])
       Consumer.HandleVTable(Class);
 
-    // Optionally warn if we're emitting a weak vtable.
-    if (Class->isExternallyVisible() &&
-        Class->getTemplateSpecializationKind() != TSK_ImplicitInstantiation) {
+    // Warn if we're emitting a weak vtable. The vtable will be weak if there is
+    // no key function or the key function is inlined. Don't warn in C++ ABIs
+    // that lack key functions, since the user won't be able to make one.
+    if (Context.getTargetInfo().getCXXABI().hasKeyFunctions() &&
+        Class->isExternallyVisible() && ClassTSK != TSK_ImplicitInstantiation) {
       const FunctionDecl *KeyFunctionDef = nullptr;
-      if (!KeyFunction || 
-          (KeyFunction->hasBody(KeyFunctionDef) && 
-           KeyFunctionDef->isInlined()))
-        Diag(Class->getLocation(), Class->getTemplateSpecializationKind() ==
-             TSK_ExplicitInstantiationDefinition 
-             ? diag::warn_weak_template_vtable : diag::warn_weak_vtable) 
-          << Class;
+      if (!KeyFunction || (KeyFunction->hasBody(KeyFunctionDef) &&
+                           KeyFunctionDef->isInlined())) {
+        Diag(Class->getLocation(),
+             ClassTSK == TSK_ExplicitInstantiationDefinition
+                 ? diag::warn_weak_template_vtable
+                 : diag::warn_weak_vtable)
+            << Class;
+      }
     }
   }
   VTableUses.clear();
diff --git a/clang/test/SemaCXX/warn-weak-vtables.cpp b/clang/test/SemaCXX/warn-weak-vtables.cpp
index d3066538715..e678f9e461e 100644
--- a/clang/test/SemaCXX/warn-weak-vtables.cpp
+++ b/clang/test/SemaCXX/warn-weak-vtables.cpp
@@ -1,5 +1,8 @@
 // RUN: %clang_cc1 %s -fsyntax-only -verify -triple %itanium_abi_triple -Wweak-vtables -Wweak-template-vtables
-// RUN: %clang_cc1 %s -fsyntax-only -triple %ms_abi_triple -Werror -Wno-weak-vtables -Wno-weak-template-vtables
+//
+// Check that this warning is disabled on MS ABI targets which don't have key
+// functions.
+// RUN: %clang_cc1 %s -fsyntax-only -triple %ms_abi_triple -Werror -Wweak-vtables -Wweak-template-vtables
 
 struct A { // expected-warning {{'A' has no out-of-line virtual method definitions; its vtable will be emitted in every translation unit}}
   virtual void f() { }