[patch] fix libstdc++/57336

[Jonathan Wakely <jwakely dot gcc at gmail dot 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.

        PR libstdc++/57336
        * include/std/functional (__invoke): Do not form function types with
        abstract return type.
        * testsuite/20_util/reference_wrapper/invoke-3.cc: New.

Tested x86_64-linux, committed to trunk.

Jakub, this is a regression against 4.8.0, should it go on the branch too?
The change is small and isolated to code used by std::reference_wrapper.

Attachment: patch.txt
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