Apply LWG 2017.  This is a only a documentation change.

llvm-svn: 191140
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: b76e6a95a9a16df6528e4136f92c9ea61cf1c830
diff --git a/include/functional b/include/functional
index fdb8e0f..c3c713e 100644
--- a/include/functional
+++ b/include/functional
@@ -56,7 +56,7 @@
 
     // invoke
     template <class... ArgTypes>
-      typename result_of<T(ArgTypes...)>::type
+      typename result_of<T&(ArgTypes&&...)>::type
           operator() (ArgTypes&&...) const;
 };