No error reported when a generic parameter doesn't meet the requirement of an associated type

[yshui]

Code

I tried this code:

trait A {
    type T: Iterator<Item = i32>;
}

trait B {
    fn x(_: impl A<T = impl Send>);
}

I expected to see this happen:

error[E0277]: `impl Send` is not an iterator

Instead, this happened:

Code compiles

Version it worked on

It most recently worked on: 1.48.0-beta.8

Version with regression

rust nightly 2020-11-08 (likely not the earliest)

[jonas-schievink]

cc @matthewjasper (might be caused by the projection bounds changes)

[camelid]

Note: not just impl Trait; also occurs with ordinary trait bounds.

[camelid]

searched nightlies: from nightly-2020-10-01 to nightly-2020-11-09
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: x86_64-apple-darwin
Reproduce with:

cargo bisect-rustc --preserve --regress=success --start=2020-10-01 

---

Indeed caused by #73905.