[Bug c++/105111] New: Ambiguous constructor overload with requires constraint

[Bug c++/105111] New: Ambiguous constructor overload with requires constraint

[andrei.popa105 at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105111

            Bug ID: 105111
           Summary: Ambiguous constructor overload with requires
                    constraint
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andrei.popa105 at yahoo dot com
  Target Milestone: ---

Let's consider the following code:

#include <iostream>
#include <type_traits>

struct S {
    template <typename T> explicit S(T) noexcept requires
std::is_signed<T>::value {
        std::cout << "T\n";
    }
    template<typename T> explicit S(const T&) noexcept {
        std::cout << "const T&\n";
    }
};

int main() {
    S s(4);
}

To compile this code I used the following command and I provide also a link to
a godbolt instance:

g++ -std=c++23 example.cpp -o example

https://godbolt.org/z/vaGafvKcT

This code doesn't compile with gcc 12.0.
I think this is a bug because the most constrained constructor is the first one
and has a matching requires clause when we create the s object.

[Bug c++/105111] Ambiguous constructor overload with requires constraint

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105111

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Andrei-Edward Popa from comment #0)
> I think this is a bug because the most constrained constructor is the first
> one and has a matching requires clause when we create the s object.

According to the standard (http://eel.is/c++draft/temp.func.order#6), "more
constrained" is used as a tiebreaker between two functions only if their
(function and template) parameter lists are equivalent.  In your example the
function parameter lists different ('T' vs 'const T&'), so I believe GCC is
correct to reject the call as ambiguous.  If other compilers accept it, it's
probably because they don't implement wg21.link/p2113r0 (which changed this
part of the standard).

*** This bug has been marked as a duplicate of bug 96333 ***

[Bug c++/105111] Ambiguous constructor overload with requires constraint

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105111

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
FWIW one workaround could be to consistently use 'T' or 'const T&' as the first
function parameter for both functions.

Another workaround could be to constrain the second overload with
'(!std::is_signed<T>::value)'.

[Bug c++/105111] Ambiguous constructor overload with requires constraint

[andrei.popa105 at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105111

--- Comment #3 from Andrei-Edward Popa <andrei.popa105 at yahoo dot com> ---
Thanks for that reference to the standard, it is clear to me why gcc reject
this call as ambiguous.