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From: "arthur.j.odwyer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/102419] New: [concepts] [regression]
 return-type-requirement of "Y<typename T::type>" does not check that T::type
 actually exists
Date: Mon, 20 Sep 2021 21:32:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102419

            Bug ID: 102419
           Summary: [concepts] [regression] return-type-requirement of
                    "Y<typename T::type>" does not check that T::type
                    actually exists
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arthur.j.odwyer at gmail dot com
  Target Milestone: ---

// https://godbolt.org/z/GWjYYnrnM
template<class, class> concept Y =3D true;
template<class T> concept X =3D requires {
    { 1 } -> Y<typename T::type>;
};
static_assert(!X<int>);


<source>:8:15: error: static assertion failed
    8 | static_assert(!X<int>);
      |               ^~~~~~~

Clang and MSVC both appear to have the correct behavior -- or what I believ=
e to
be the consistent/useful/majority behavior, anyway -- which is that since
T::type doesn't exist, the concept shouldn't be satisfied.

This seems to be a regression; according to Godbolt, GCC 10.3 had the corre=
ct
behavior but GCC 11.1 lost it.=20

I wonder if #92268 could be related somehow, since it seems to be something
like the inverse issue (nonexistent nested type causing a hard error in 10.=
x),
and it was marked fixed presumably somewhere in the 11.x timeframe.=