impl-trait type alias is usable in impl blocks

[DutchGhost]

As requested in #53345 (comment), here's a follow up.

The following code below implements the trait MyTrait for an alias Bar, which is an alias to MyTrait. It doesnt really make sense to implement a trait...for that same trait.

type_alias_impl_trait doesn't mention anything about this, however the previous existential_type notes:

One last difference between existential type aliases and normal type aliases is that existential type aliases cannot be used in impl blocks

#![feature(type_alias_impl_trait)]

trait MyTrait {}

impl MyTrait for () {}

type Bar = impl MyTrait;

impl MyTrait for Bar {}

fn bazr() -> Bar { }

fn main() {}

Im not sure this should be allowed.

cc @Aaron1011 @nikomatsakis @Centril

[jackh726]

This is fixed on master.