[Bug c++/59204] New: Incorrect metaprogram evaluation in SFINAE context

[webrown.cpp at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59204

            Bug ID: 59204
           Summary: Incorrect metaprogram evaluation in SFINAE context
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: webrown.cpp at gmail dot com

Created attachment 31255
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31255&action=edit
Demonstrating SFINAE failure to fail

Command:  g++-mp-4.9 -O3 -std=gnu++1y bug2.cc
Version: g++-mp-4.9 (MacPorts gcc49 4.9-20131110_0) 4.9.0 20131110
(experimental)

Attached program fails to compile because the 2nd static_assert incorrectly
leads to a diagnostic.

Used in a SFINAE context, it seems that the type expression
  void_t<typename T::type>
always yields the type void.  This seems too aggressive an outcome; when T has
no T::type member, the expression should instead be treated as invalid, hence
its context rejected due to SFINAE.