Allow consteval functions to return types by using the declarator typename auto. These functions would be evaluated at compile time and usable anywhere a type is expected.

Motivation

Express compile-time type computation using normal control flow.
Zero runtime cost since consteval are deduced at compile time.

Rules (proposal)

Must be declared consteval.
Each return must produce a type expression (e.g. int, std::vector<T>, decltype(expr)).
Usable in using, declarations, template arguments, concepts, etc.

Why

More readable than alias templates/struct metafunctions for many cases.
Aligns type-level programming with existing constexpr/consteval function model

Current syntax:

template<int N>

struct select_type {

using type = std::conditional_t<N == 0, int, double>;

};

using T = typename select_type<1>::type; // T == double

Proposed syntax:

consteval typename auto select_type(int n) {

return n == 0 ? int : double;

}

or using the trailing type syntax

consteval auto select_type(int n) -> typename auto {

return n == 0 ? int : double;

}

using T = select_type(1); // T == double

More examples

template<typename T>

consteval typename auto make_pointer_if(bool b) {

return b ? T* : T;

}

using A = make_pointer_if<int>(true); // int*

using B = make_pointer_if<int>(false); // int

tell me what you think.