Index: cfe/trunk/lib/Sema/SemaTemplate.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaTemplate.cpp
+++ cfe/trunk/lib/Sema/SemaTemplate.cpp
@@ -4434,7 +4434,7 @@
 
   // If the argument type is dependent, instantiate it now based
   // on the previously-computed template arguments.
-  if (ArgType->getType()->isDependentType()) {
+  if (ArgType->getType()->isInstantiationDependentType()) {
     Sema::InstantiatingTemplate Inst(SemaRef, TemplateLoc,
                                      Param, Template, Converted,
                                      SourceRange(TemplateLoc, RAngleLoc));
Index: cfe/trunk/test/SemaCXX/alias-template.cpp
===================================================================
--- cfe/trunk/test/SemaCXX/alias-template.cpp
+++ cfe/trunk/test/SemaCXX/alias-template.cpp
@@ -179,3 +179,13 @@
 };
 static_assert(__is_same(S<3>::U, X[2]), ""); // expected-error {{static_assert failed}}
 }
+
+namespace PR39623 {
+template <class T>
+using void_t = void;
+
+template <class T, class = void_t<typename T::wait_what>>
+int sfinae_me() { return 0; } // expected-note{{candidate template ignored: substitution failure}}
+
+int g = sfinae_me<int>(); // expected-error{{no matching function for call to 'sfinae_me'}}
+}