4 files changed, 19 insertions, 2 deletions
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 46f6106c13a..dcb747fe8d7 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -2391,8 +2391,14 @@ static DeclContext *getPrimaryContextForMerging(DeclContext *DC) {
   if (NamespaceDecl *ND = dyn_cast<NamespaceDecl>(DC))
     return ND->getOriginalNamespace();
 
+  // There is one tricky case here: if DC is a class with no definition, then
+  // we're merging a declaration whose definition is added by an update record,
+  // but we've not yet loaded that update record. In this case, we use the
+  // canonical declaration for merging until we get a real definition.
+  // FIXME: When we add a definition, we may need to move the partial lookup
+  // information from the canonical declaration onto the chosen definition.
   if (CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(DC))
-    return RD->getDefinition();
+    return RD->getPrimaryContext();
 
   if (EnumDecl *ED = dyn_cast<EnumDecl>(DC))
     return ED->getASTContext().getLangOpts().CPlusPlus? ED->getDefinition()
@@ -3361,7 +3367,12 @@ void ASTDeclReader::UpdateDecl(Decl *D, ModuleFile &ModuleFile,
           Reader.ReadTemplateArgumentList(TemplArgs, F, Record, Idx);
           auto *TemplArgList = TemplateArgumentList::CreateCopy(
               Reader.getContext(), TemplArgs.data(), TemplArgs.size());
-          Spec->setInstantiationOf(PartialSpec, TemplArgList);
+
+          // FIXME: If we already have a partial specialization set,
+          // check that it matches.
+          if (!Spec->getSpecializedTemplateOrPartial()
+                   .is<ClassTemplatePartialSpecializationDecl *>())
+            Spec->setInstantiationOf(PartialSpec, TemplArgList);
         }
       }
 
diff --git a/clang/test/Modules/Inputs/cxx-templates-a.h b/clang/test/Modules/Inputs/cxx-templates-a.h
index 1ade811d71d..f3150398e7a 100644
--- a/clang/test/Modules/Inputs/cxx-templates-a.h
+++ b/clang/test/Modules/Inputs/cxx-templates-a.h
@@ -58,6 +58,7 @@ template<typename T> struct WithPartialSpecialization<T*> {
   T &f() { static T t; return t; }
 };
 typedef WithPartialSpecializationUse::type WithPartialSpecializationInstantiate;
+typedef WithPartialSpecialization<void(int)>::type WithPartialSpecializationInstantiate2;
 
 template<> struct WithExplicitSpecialization<int> {
   int n;
diff --git a/clang/test/Modules/Inputs/cxx-templates-b.h b/clang/test/Modules/Inputs/cxx-templates-b.h
index 77a2c2333ac..8b97d22a370 100644
--- a/clang/test/Modules/Inputs/cxx-templates-b.h
+++ b/clang/test/Modules/Inputs/cxx-templates-b.h
@@ -57,6 +57,8 @@ template<int> struct UseInt;
 template<typename T> void UseRedeclaredEnum(UseInt<T() + CommonTemplate<char>::a>);
 constexpr void (*UseRedeclaredEnumB)(UseInt<1>) = UseRedeclaredEnum<int>;
 
+typedef WithPartialSpecialization<void(int)>::type WithPartialSpecializationInstantiate3;
+
 template<typename> struct MergeSpecializations;
 template<typename T> struct MergeSpecializations<T&> {
   typedef int partially_specialized_in_b;
@@ -76,4 +78,5 @@ void TriggerInstantiation() {
   UseDefinedInBImpl<void>();
   Std::f<int>();
   PartiallyInstantiatePartialSpec<int*>::foo();
+  WithPartialSpecialization<void(int)>::type x;
 }
diff --git a/clang/test/Modules/Inputs/cxx-templates-common.h b/clang/test/Modules/Inputs/cxx-templates-common.h
index 074cc8f9007..f7c083249f8 100644
--- a/clang/test/Modules/Inputs/cxx-templates-common.h
+++ b/clang/test/Modules/Inputs/cxx-templates-common.h
@@ -32,7 +32,9 @@ template<typename T> struct Outer {
 };
 
 template<typename T> struct WithPartialSpecialization {};
+template<typename T> struct WithPartialSpecialization<void(T)> { typedef int type; };
 typedef WithPartialSpecialization<int*> WithPartialSpecializationUse;
+typedef WithPartialSpecialization<void(int)> WithPartialSpecializationUse2;
 
 template<typename T> struct WithExplicitSpecialization;
 typedef WithExplicitSpecialization<int> WithExplicitSpecializationUse;