On Mon, 27 Oct 2025 at 13:02, Frederick Virchanza Gotham via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On Mon, Oct 27, 2025 at 12:13 PM Jonathan Wakely wrote:
> >
> > This is problematic for arbitrary types, which might not be default
> constructible. In generic code, you don't know how to construct a value to
> return.
> >
> > But you can return std::type_identity<int>{} or
> std::type_identity<double>{} and then use decltype(select_type<1>())::type.
>
> #include <type_traits>
> #include <semaphore>
>
> template<int N> consteval auto select_type_detail(void)
> {
> /**/ if constexpr ( 0 == N ) return (int
> *)nullptr;
> else if constexpr ( 1 == N ) return (double
> *)nullptr;
> else if constexpr ( 2 == N ) return
> (std::binary_semaphore*)nullptr;
>

You can't use this to return reference types (nor abominable functions
types, although that's less of a problem in practice).


> }
>
> template<int N>
> using select_type = std::remove_pointer_t< decltype(
> select_type_detail<N>() ) >;
>
> #include <iostream>
> #include <typeinfo>
>
> int main(void)
> {
> select_type<2> var(0);
> std::cout << typeid(var).name() << std::endl;
> }
> --
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