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[v3 PATCH] PR libstdc++/77288 and the newest proposed resolution for LWG 2756


Tested on Linux-x64.

N.B. I know that the assignment operators have constraints in template
parameters, I want to move them to the return type separately.

N.B2. This fixes the bug for std::optional only; I plan to port the fix
to std::experimental::optional in a separate patch, and then backport
that separate fix to 6.x.

N.B3. The newest p/r for LWG 2756 is not up yet in the issue list,
and hasn't been looked at by LEWG/LWG, but neither was the previous
one. :) Nevertheless, this new p/r and the patch are a massive
improvement, and keeps
existing code and expectations working, makes all sorts of interesting types
work, and keeps the conversion functionality.

2016-08-23  Ville Voutilainen  <ville.voutilainen@gmail.com>

    PR libstdc++/77288
    * include/std/optional (__is_optional_impl, __is_optional): Remove.
    (__converts_from_optional, __assigns_from_optional): New.
    (optional(_Up&&)): Use is_same instead of __is_optional.
    (optional(const optional<_Up>&)): Constrain with
    __converts_from_optional.
    (optional(optional<_Up>&&)): Likewise.
    (operator=(_Up&&)): Use is_same instead of __is_optional.
    (operator=(const optional<_Up>&)): Constrain with
    __converts_from_optional and __assigns_from_optional.
    (operator=(optional<_Up>&&)): Likewise.
    * testsuite/20_util/optional/77288.cc: New.
    * testsuite/20_util/optional/cons/value.cc: Adjust.

Attachment: lwg2756_2.diff
Description: Text document


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