Type of const generic can't be associated type.

I tried this code:

struct Thing;

trait Trait {
    type Assoc;
}
impl Trait for Thing {
    type Assoc = i32;
}

type Alias1 = i32;
type Alias2 = <Thing as Trait>::Assoc;

fn works<const N: Alias1>() {}
fn fails<const N: Alias2>() {}

I expected works and fails to either both compile or both error. Instead, only works compiles:

error: `<Thing as Trait>::Assoc` is forbidden as the type of a const generic parameter
  --> src/lib.rs:14:19
   |
14 | fn fails<const N: Alias2>() {}
   |                   ^^^^^^
   |
   = note: the only supported types are integers, `bool`, and `char`

See also https://github.com/rust-lang/rust/issues/149203

Meta

Reproducible on the playground with version 1.93.0-nightly (2025-11-21 27b076af7e3e7a363975)

Start with the reproducer in the issue and compare how the compiler handles Alias1 and Alias2 as const generic parameter types. Investigate the const-generic type validation involved, then verify that equivalent aliases are either both accepted or both rejected using the shown example.