repeated TAIT doesn't detect lifetime conflict

[nikomatsakis]

This code compiles, but it should not:

#![feature(min_type_alias_impl_trait)]

type X<'a, 'b> = impl std::fmt::Debug;

fn f<'t, 'u>(a: &'t u32, b: &'u u32) -> (X<'t, 'u>, X<'u, 't>) {
    (a, a)
}

The function creates two constraints:

• X<'t, 'u> = &'t u32, which means type X<'a, 'b> = &'a u32
• X<'u, 't> = &'t u32, which means type X<'a, 'b> = &'b u32

These should not both be satisfiable!

[nikomatsakis]

@rustbot assign @spastorino

[nikomatsakis]

@rustbot assign @nikomatsakis

I have been working on this. It might also make sense to assign to @oli-obk instead though.

[nikomatsakis]

@rustbot assign @oli-obk

[nikomatsakis]

In progress PR: #86410