The additional logic added to __is_convertible_helper in order to
support is_nothrow_convertible makes some uses of is_convertible
ill-formed. This appears to be due to PR c++/87603, but can be avoided
just by defining a separate helper for is_nothrow_convertible. The same
problems are likely to still exist for is_nothrow_convertible, but that
is new and so won't cause regressions for existing users of
is_convertible.
PR libstdc++/89194
* include/std/type_traits (__is_convertible_helper)
(__is_convertible_helper<_From, _To, false>): Revert changes to
support is_nothrow_convertible.
(__is_nt_convertible_helper): New helper.
(is_nothrow_convertible): Use __is_nt_convertible_helper.
From-SVN: r268543
