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From: "jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/104111] Concept evaluation depends on context where it was
 first checked
Date: Fri, 27 May 2022 17:58:21 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104111

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |67491

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Patrick Palka from comment #3)
> > because concept-id evaluation shall not depend on the context.
>=20
> One consequence of making this change to concept-id evaluation would be t=
hat
> for:
>=20
>   template<class T> void f() requires (!C<T>);
>=20
> during constraint checking for say f<int>(), we no longer evaluate C<int>
> (as part of evaluation of the atomic constraint !C<T>) in the access cont=
ext
> of f, which seems surprising to me.
>=20
> CC'ing Jason for guidance.

This issue was discussed on the CWG mailing list back in 2018, but seems ne=
ver
to have made it to the issues list.  There was general agreement at the time
that access should be checked in the lexical context of the atomic constrai=
nt,
as with other expressions; this does indeed have the consequence that you
mention.  Which means that since we don't have class-scope concepts, any
constraints that need to depend on access control need to be written direct=
ly
in the requires-clause rather than through a concept.  Or just give up on
trying to express constraints that depend on access.

An alternative fix for this bug would be to include the evaluation context =
in
the satisfaction cache.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D67491
[Bug 67491] [meta-bug] concepts issues=