diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 5d4caacd8124..d458050585f1 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -2912,16 +2912,6 @@ bool Sema::CheckTemplateArgumentList(TemplateDecl *Template,
     // arguments, just break out now and we'll fill in the argument pack below.
     if ((*Param)->isTemplateParameterPack())
       break;
-
-    // If our template is a template template parameter that hasn't acquired
-    // its proper context yet (e.g., because we're using the template template
-    // parameter in the signature of a function template, before we've built
-    // the function template itself), don't attempt substitution of default
-    // template arguments at this point: we don't have enough context to
-    // do it properly.
-    if (isTemplateTemplateParameter && 
-        Template->getDeclContext()->isTranslationUnit())
-      break;
     
     // We have a default template argument that we will use.
     TemplateArgumentLoc Arg;
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 3c1964175b10..69e592ee2535 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -62,8 +62,20 @@ Sema::getTemplateInstantiationArgs(NamedDecl *D,
   if (!Ctx) {
     Ctx = D->getDeclContext();
     
-    assert((!D->isTemplateParameter() || !Ctx->isTranslationUnit()) &&
-           "Template parameter doesn't have its context yet!");
+    // If we have a template template parameter with translation unit context,
+    // then we're performing substitution into a default template argument of
+    // this template template parameter before we've constructed the template
+    // that will own this template template parameter. In this case, we
+    // use empty template parameter lists for all of the outer templates
+    // to avoid performing any substitutions.
+    if (Ctx->isTranslationUnit()) {
+      if (TemplateTemplateParmDecl *TTP 
+                                      = dyn_cast<TemplateTemplateParmDecl>(D)) {
+        for (unsigned I = 0, N = TTP->getDepth() + 1; I != N; ++I)
+          Result.addOuterTemplateArguments(0, 0);
+        return Result;
+      }
+    }
   }
   
   while (!Ctx->isFileContext()) {
diff --git a/clang/test/SemaTemplate/default-arguments.cpp b/clang/test/SemaTemplate/default-arguments.cpp
index 9ea0fc2e4d52..6391369aa5c4 100644
--- a/clang/test/SemaTemplate/default-arguments.cpp
+++ b/clang/test/SemaTemplate/default-arguments.cpp
@@ -121,3 +121,18 @@ X6<long, X5b> x6c;
 
 
 template<template<class> class X = B<int> > struct X7; // expected-error{{must be a class template}}
+
+namespace PR9643 {
+  template<typename T> class allocator {};
+  template<typename T, typename U = allocator<T> > class vector {};
+
+  template<template<typename U, typename = allocator<U> > class container,
+           typename DT>
+  container<DT> initializer(const DT& d) {
+    return container<DT>();
+  }
+
+  void f() {
+    vector<int, allocator<int> > v = initializer<vector>(5);
+  }
+}