[SUGGESTION] Requires clause for aliases

[JohelEGP]

A natural extension to 63efa6e.

Will your feature suggestion eliminate X% of security vulnerabilities of a given kind in current C++ code? No.

Will your feature suggestion automate or eliminate X% of current C++ guidance literature?

(Make it possible to) make the default:

• I.9: If an interface is a template, document its parameters using concepts
• T.10: Specify concepts for all template arguments

No longer teach:

• T.48: If your compiler does not support concepts, fake them with enable_if
• T.121: Use template metaprogramming primarily to emulate concepts

Describe alternatives you've considered.
https://cpp2.godbolt.org/z/Mr9TMo7xG:

i: <T> type
  // requires std::integral<T>
  == std::enable_if<
        std::integral<T>,
        std::make_signed<T>
      >::type::type;

[JohelEGP]

This is better dealt with after #486.

A test: https://cpp2.godbolt.org/z/bxsvexdxr.

integer_t: <T> type requires std::integral<T>           == T;
integer_v: <V: _> _ requires std::integral<decltype(V)> == V;
main: () = {
  std::ignore = integer_t<u32>(integer_v<:u32                        = 0>);
  static_assert(!std::invocable<decltype(:<T> (x: T) -> integer_t<T> = {}), std::nullptr_t>);
  static_assert(!std::invocable<decltype(:<T> (x: T) -> std::type_identity_t<decltype(integer_v<:T = nullptr>)> = {}),
                                         std::nullptr_t>);
}