[patch] fix libstdc++/57336

[Daniel Krügler]

2013/5/21 Jonathan Wakely <jwakely.gcc@gmail.com>:
> This broke due to some front end changes that disallow forming
> function types that return abstract types.  std::reference_wrapper
> always passes an lvalue reference to __invoke so it's correct to use
> an lvalue reference as the result_of's template argument.

I agree. This was

http://cplusplus.github.io/LWG/lwg-defects.html#2017

wasn't it?

- Daniel