[Bug c++/91466] [concepts] indicates "used in its own initializer" when not, constraint order change passes compilation.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91466

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |10.0
             Blocks|                            |67491

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Compiles on current trunk after changing the return type constraint:

#include <concepts>
#include <array>

template<typename T>
concept integral_constant_ = std::is_empty_v<T> && std::is_trivial_v<T> &&
requires
{
    typename T::value_type;
    requires std::is_integral_v<typename T::value_type>;
    { T::value } -> std::same_as<typename T::value_type>;
};

struct sz_fn
{
template<typename R> requires requires(R&& r) { {static_cast<R&&>(r).size()} ->
integral_constant_; }
constexpr auto operator()(R&& r) {
    return static_cast<R&&>(r).size();
}
};

constexpr auto sz = sz_fn{};


int main()
{
    auto arr = std::array<int, 5>{1, 2, 3, 4, 5};

    return !std::is_invocable_v<decltype(sz), decltype(arr)>;
}


Referenced Bugs:

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