Multiple defining uses of type alias impl trait in single function are not handled

[matthewjasper]

I tried this code:

#![feature(type_alias_impl_trait)]

type X<A, B> = impl Into<&'static A>;

fn f<A, B: 'static>(a: &'static A, b: B) -> (X<A, B>, X<B, A>) {
    (a, a)
}

fn main() {
    println!("{}", <X<_, _> as Into<&String>>::into(f(&[1isize, 2, 3], String::new()).1));
}

I expected to see this happen: compiler error because X<A, B> is unified with A and also X<B, A> is unified with A.

Instead, this happened: program compiles, runs and segfaults.

Meta

rustc --version --verbose:

rustc 1.46.0-nightly (feb3536eb 2020-06-09)
binary: rustc
commit-hash: feb3536eba10c2e4585d066629598f03d5ddc7c6
commit-date: 2020-06-09
host: x86_64-unknown-linux-gnu
release: 1.46.0-nightly
LLVM version: 10.0

[matthewjasper]

Variants on this can cause ICEs and llvm errors instead, as one might expect.

[nikomatsakis]

@rustbot assign @spastorino

Santiago is working on the PR for this.