const_generics with default arguments

[lcnr]

Should the following code be accepted?

// compiles rn
#![feature(const_generics)]

struct A<T = u32, const N: usize> {
    arg: T,
}

type C = A<u32, 7>;

if so, we probably also want to accept the following:

// this currently fails
type B = A<7>;
//~^ ERROR constant provided when a type was expected

playground

cc @varkor @eddyb

[rodrimati1992]

So,how would the following be handled?

#![feature(const_generics)]

struct A<T = u32, const N: usize> {
    arg: T,
}

type C = u64;
const C: usize = 7;

const _: [A<C>; 0] = [];

That is Foo<C> (passing an identifier as a const argument).

[lcnr]

@rodrimati1992 A<C> should be equal to A<u32; C> imo.

rn literals which are in both the type and the value namespace are always used as type params though (see #66615). So this would fail to compile even if we get A<10> to work.