Index: include/llvm/ADT/STLExtras.h
===================================================================
--- include/llvm/ADT/STLExtras.h
+++ include/llvm/ADT/STLExtras.h
@@ -117,7 +117,7 @@
           iterator_category;
   typedef typename std::iterator_traits<RootIt>::difference_type
           difference_type;
-  typedef typename UnaryFunc::result_type value_type;
+  typedef decltype(Fn(*current)) value_type;
 
   typedef void pointer;
   //typedef typename UnaryFunc::result_type *pointer;