4 files changed, 19 insertions, 2 deletions

diff --git a/clang/lib/CodeGen/CGClass.cpp b/clang/lib/CodeGen/CGClass.cpp
index 5b4dc5ff0ab..127d7df348e 100644
--- a/clang/lib/CodeGen/CGClass.cpp
+++ b/clang/lib/CodeGen/CGClass.cpp
@@ -2770,10 +2770,19 @@ CodeGenFunction::CanDevirtualizeMemberFunctionCall(const Expr *Base,
 
   // We can devirtualize calls on an object accessed by a class member access
   // expression, since by C++11 [basic.life]p6 we know that it can't refer to
-  // a derived class object constructed in the same location.
+  // a derived class object constructed in the same location. However, we avoid
+  // devirtualizing a call to a template function that we could instantiate
+  // implicitly, but have not decided to do so. This is needed because if this
+  // function does not get instantiated, the devirtualization will create a
+  // direct call to a function whose body may not exist. In contrast, calls to
+  // template functions that are not defined in this TU are allowed to be 
+  // devirtualized under assumption that it is user responsibility to
+  // instantiate them in some other TU.
   if (const MemberExpr *ME = dyn_cast<MemberExpr>(Base))
     if (const ValueDecl *VD = dyn_cast<ValueDecl>(ME->getMemberDecl()))
-      return VD->getType()->isRecordType();
+      return VD->getType()->isRecordType() && 
+             (MD->instantiationIsPending() || MD->isDefined() ||
+               !MD->isImplicitlyInstantiable());
 
   // Likewise for calls on an object accessed by a (non-reference) pointer to
   // member access.
diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index e7b0914641f..007a5e483e6 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -740,6 +740,9 @@ void Sema::ActOnEndOfTranslationUnit() {
       // Load pending instantiations from the external source.
       SmallVector<PendingImplicitInstantiation, 4> Pending;
       ExternalSource->ReadPendingInstantiations(Pending);
+      for (auto PII : Pending)
+        if (auto Func = dyn_cast<FunctionDecl>(PII.first))
+          Func->setInstantiationIsPending(true);
       PendingInstantiations.insert(PendingInstantiations.begin(),
                                    Pending.begin(), Pending.end());
     }
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 75a6903392e..f49df6b3216 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -13732,6 +13732,7 @@ void Sema::MarkFunctionReferenced(SourceLocation Loc, FunctionDecl *Func,
         // call to such a function.
         InstantiateFunctionDefinition(PointOfInstantiation, Func);
       else {
+        Func->setInstantiationIsPending(true);
         PendingInstantiations.push_back(std::make_pair(Func,
                                                        PointOfInstantiation));
         // Notify the consumer that a function was implicitly instantiated.
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 148ce24293a..c58122cd271 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -3782,6 +3782,7 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
       // Try again at the end of the translation unit (at which point a
       // definition will be required).
       assert(!Recursive);
+      Function->setInstantiationIsPending(true);
       PendingInstantiations.push_back(
         std::make_pair(Function, PointOfInstantiation));
     } else if (TSK == TSK_ImplicitInstantiation) {
@@ -3801,6 +3802,7 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
   // Postpone late parsed template instantiations.
   if (PatternDecl->isLateTemplateParsed() &&
       !LateTemplateParser) {
+    Function->setInstantiationIsPending(true);
     PendingInstantiations.push_back(
       std::make_pair(Function, PointOfInstantiation));
     return;
@@ -5146,6 +5148,8 @@ void Sema::PerformPendingInstantiations(bool LocalOnly) {
                                 TSK_ExplicitInstantiationDefinition;
       InstantiateFunctionDefinition(/*FIXME:*/Inst.second, Function, true,
                                     DefinitionRequired, true);
+      if (Function->isDefined())
+        Function->setInstantiationIsPending(false);
       continue;
     }