[Bug c++/104873] Bug in overload resolution for constrained class templates with deduction guides

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104873

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Reduced testcase:

template<class T> concept C = true;

template<class T> requires C<T>
struct S {
  S(...);
};

template<class T> S(S<T>) -> S<S<T>>;

using type = decltype(S(S<int>()));
using type = S<int>; // Clang accepts, GCC demands S<S<int>>


The difference is that the copy deduction candidate generated by GCC is
unconstrained:
  template<class T> S(S<T>) -> S<T>;
whereas Clang presumably attaches the constraints of the class template to the
candidate:
  template<class T> requires C<T> S(S<T>) -> S<T>;


This also occurs for guides synthesized from a constructor:

template<class T> concept C = true;

template<class T> requires C<T>
struct S {
  S();
  S(T); // #1
};

template<class T> S(T) -> S<S<T>>; // #2

using type = decltype(S(0)); // Clang selects guide for #1, GCC selects #2
using type = S<int>;         // therefore Clang accepts, GCC demands S<S<int>>


Is it true that a deduction guide inherits the constraints of the class
template?  It's not clear to me according to [over.match.class.deduct].