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From: "jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/102419] [11/12 Regression][concepts]
 return-type-requirement of "Y<typename T::type>" does not check that T::type
 actually exists
Date: Sun, 27 Mar 2022 19:21:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102419

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org
            Summary|[11/12                      |[11/12
                   |Regression][concepts]       |Regression][concepts]
                   |[regression]                |return-type-requirement of
                   |return-type-requirement of  |"Y<typename T::type>" does
                   |"Y<typename T::type>" does  |not check that T::type
                   |not check that T::type      |actually exists
                   |actually exists             |

--- Comment #6 from Jason Merrill <jason at gcc dot gnu.org> ---
Concept satisfaction was very deliberately designed in committee discussion=
 to
work differently from void_t, based on the normalized form rather than the
concept-id as written.  So this example is well-formed:

template <class Tz> concept is_void =3D same_as(Tz, void);
template <class Ta, class Tb> concept void_or_same =3D is_void<Ta> || same_=
as<Ta,
Tb>;
template <class T> void f() requires void_or_same<T,T&>;
int main() { f<void>(); } // OK

It definitely is an inconsistency with non-concept template-ids, but that's=
 the
design.

C++20 national body comment CA104
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p2103r0.html#CA104=
) is
about one situation where we do need to substitute directly into the argume=
nts,
but it is a single exception to the rule.

In the discussion of CA104 I suggested that we might want to reconsider this
design (on 2019-11-08, if you want to look up the reflector message), and m=
ake
my example above ill-formed, but we ended up making an exception instead.

The behavior change you are seeing is from properly implementing
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1452r2.html and no=
t a
bug.=