[libcxx] Fix PR22771 - Support access control SFINAE in the library
version of is_convertible.
Summary:
Currently the conversion check does not take place in a context where
access control SFINAE is applied. This patch changes the context of
the test expression so that SFINAE occurs if access control does not
permit the conversion.
Related bug: https://llvm.org/bugs/show_bug.cgi?id=22771
Reviewers: mclow.lists, rsmith, dim
Reviewed By: dim
Subscribers: dim, rodrigc, emaste, cfe-commits
Differential Revision: http://reviews.llvm.org/D8461
This fixes building clang, and other programs using libc++, with newer
versions of gcc (specifically, gcc 4.8 and higher).
Reported by: rodrigc
MFC after: 1 week
