[Bug libstdc++/56283] std::result_of does not gracefully SFINAE (not define ::type)

[potswa at mac dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56283

--- Comment #1 from David Krauss <potswa at mac dot com> 2013-02-11 04:57:46 UTC ---
Note, defining the trait with the expression
declval<Fn>()(declval<ArgTypes>()...) does work. Clang 3.1/libc++ behaves the
same way.