https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102419
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 = same_as(Tz, void);
template <class Ta, class Tb> concept void_or_same = 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 arguments,
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 make
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 not a
bug.
