As Tomasz pointed out on mattermost,
given
```cpp
template <class T>
struct MustBeInt {
static_assert(std::same_as<T, int>);
};
struct Poison {
template <class T>
constexpr auto operator()(T) const noexcept -> MustBeInt<T> {
return {};
}
};
std::cw<Poison{}>(std::cw<5>);
```
This should work according to the wording
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2026/p3978r3.pdf
```cpp
template<class... Args>
static constexpr decltype(auto) operator()(Args&&... args) noexcept(see below);
```
```
- Let call-expr be constant_wrapper<INVOKE (value, remove_cvref_t<Args>::value...)>{} if all types
in remove_cvref_t<Args>... satisfy constexpr-param and constant_wrapper<INVOKE (value, remove_-
cvref_t<Args>::value...)> is a valid type, otherwise let call-expr be INVOKE (value, std::forward<Args>(args)...).
- Constraints: call-expr is a valid expression.
- Effects: Equivalent to: return call-expr;
- Remarks: The exception specification is equivalent to noexcept(call-expr).
```
so basically the spec says, there are two cases
- constexpr-param case
- runtime case
and if constexpr-param case works, uses it, otherwise fallback to
runtime case if it is valid, otherwise substitution error.
In this case, `std::cw<5>` satisfy `constexpr-param`, and
`constant_wrapper<Poison{}(5)>` is valid , it should just work. however,
our implementation implemented the two cases in two overload
```cpp
template <class... _Args>
requires(__constexpr_param<remove_cvref_t<_Args>> && ...)
_LIBCPP_HIDE_FROM_ABI static constexpr auto __call(_Args&&...) noexcept
-> constant_wrapper<std::invoke(value, remove_cvref_t<_Args>::value...)> {
return {};
}
template <class... _Args>
_LIBCPP_HIDE_FROM_ABI static constexpr auto
__call(_Args&&... __args) noexcept(noexcept(std::invoke(value, std::forward<_Args>(__args)...)))
-> decltype(std::invoke(value, std::forward<_Args>(__args)...)) {
return std::invoke(value, std::forward<_Args>(__args)...);
}
```
The logic is that, both overloads use return type SFINAE and the
constexpr-param case's concept is more constrained so it will be picked
first if constexpr-param case works.
However, in this example, the second overload's return type SFINAE
causes a hard error by the `static_assert` in the user code.
The fix is simply constrain the second overload and not using return
type SFINAE so it short circuits if constexpr-param case is valid.
As a drive by, removed the internal helper function and directly make
the public interface `operator()` two overloads so that we can apply
`[[nodiscard]]` on the constexpr-param case
44633491b7