regression 1.49: unexpected unsized tail ICE

[Mark-Simulacrum]

https://crater-reports.s3.amazonaws.com/beta-1.49-1/beta-2020-11-26/gh/folex.libp2p-multilang/log.txt

@rustbot modify labels: +regression-from-stable-to-beta I-ICE

[dillona]

searched nightlies: from nightly-2020-09-24 to nightly-2020-12-11
regressed nightly: nightly-2020-10-07
searched commits: from a1dfd24 to 98edd1f
regressed commit: 08e2d46

bisected with cargo-bisect-rustc v0.6.0

Host triple: aarch64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc

[Mark-Simulacrum]

cc @matthewjasper -- seems like this one might be an actual bug #73905

[matthewjasper]

Small case that I think shows this issue, it incorrectly fails to compile on beta/nightly.

trait A {
    type B;
}
trait M {}

struct G<T, U>(*const T, *const U);

impl<T, U> Clone for G<T, U> {
    fn clone(&self) -> Self {
        G { ..*self }
    }
}

impl<T, U> Copy for G<T, U::B>
where 
T: A<B = U>,
U: A
{}

impl A for () {
    type B = ();
}

fn is_m<T: M>(_: T) {}

fn main() {
    let x = G(&(), &());
    drop(x);
    drop(x);
}

[fanninpm]

Playground link of previous example

N.B. I haven't gotten it to ICE, so I can't add it to the glacier.