Index: lib/Sema/TreeTransform.h
===================================================================
--- lib/Sema/TreeTransform.h
+++ lib/Sema/TreeTransform.h
@@ -3311,8 +3311,6 @@
         if (Out.isInvalid())
           return true;
 
-        // FIXME: Can this happen? We should not try to expand the pack
-        // in this case.
         if (Out.get()->containsUnexpandedParameterPack()) {
           Out = getDerived().RebuildPackExpansion(
               Out.get(), Expansion->getEllipsisLoc(), OrigNumExpansions);
@@ -4781,6 +4779,13 @@
           if (NewType.isNull())
             return true;
 
+          if (NewType->containsUnexpandedParameterPack()) {
+            NewType = getDerived().RebuildPackExpansionType(
+                NewType, SourceRange(), Loc, NumExpansions);
+            if (NewType.isNull())
+              return true;
+          }
+
           if (ParamInfos)
             PInfos.set(OutParamTypes.size(), ParamInfos[i]);
           OutParamTypes.push_back(NewType);
Index: test/CXX/temp/temp.decls/temp.variadic/p5.cpp
===================================================================
--- test/CXX/temp/temp.decls/temp.variadic/p5.cpp
+++ test/CXX/temp/temp.decls/temp.variadic/p5.cpp
@@ -437,3 +437,32 @@
   template void g<>();
   template void g<1, 2, 3>();
 }
+
+template <class... Ts>
+int var_expr(Ts... ts);
+
+template <class... Ts>
+auto a_function(Ts... ts) -> decltype(var_expr(ts...));
+
+template <class T>
+using partial = decltype(a_function<int, T>);
+
+int use_partial() { partial<char> n; }
+
+namespace PR26017 {
+template <class T>
+struct Foo {};
+template <class... Ts>
+using FooAlias = Foo<void(Ts...)>;
+
+template <class... Ts>
+void bar(const FooAlias<Ts...> &) {}
+
+int fn() {
+  FooAlias<> a;
+  bar(a);
+
+  FooAlias<int> b;
+  bar(b);
+}
+}