lazy type alias: type parameters are not considered constrained in impls

[fmease]

Uplifted from #112792 (comment).

#![feature(lazy_type_alias)]
#![allow(incomplete_features)]

type Faa<A> = Foo<A>;
struct Foo<A>(A);

impl<A> Faa<A> {}

error[E0207]: the type parameter `A` is not constrained by the impl trait, self type, or predicates
 --> src/lib.rs:11:6
  |
7 | impl<A> Faa<A> {}
  |      ^ unconstrained type parameter