Specifying one associated type makes Rust forget about the constraint on the other ones

[Ten0]

I tried this code (which is actually the minimum reproduction I could find, as the bug seems to disappear when I try to reduce it more):

use std::marker::PhantomData;

pub struct XImpl<T, E, F2, F1>
where
	F2: Fn(E),
{
	f1: F1,
	f2: F2,
	_ghost: PhantomData<(T, E)>,
}

pub trait X<T>: Sized {
	type F1;
	type F2: Fn(Self::E);
	type E;

	fn and<NewF1, NewF1Generator>(self, f: NewF1Generator) -> XImpl<T, Self::E, Self::F2, NewF1>
	where
		NewF1Generator: FnOnce(Self::F1) -> NewF1;
}

impl<T, E, F2, F1> X<T> for XImpl<T, E, F2, F1>
where
	F2: Fn(E),
{
	type E = E;
	type F2 = F2;
	type F1 = F1;

	fn and<NewF1, NewF1Generator>(self, f: NewF1Generator) -> XImpl<T, E, F2, NewF1>
	where
		NewF1Generator: FnOnce(F1) -> NewF1,
	{
		XImpl {
			f1: f(self.f1),
			f2: self.f2,
			_ghost: PhantomData,
		}
	}
}

fn f() -> impl X<()> {
	XImpl {
		f1: || (),
		f2: |()| (),
		_ghost: PhantomData,
	}
}

fn f2() -> impl X<()> {
	f().and(|rb| rb)
}

It does compile: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=54e239ced207241c8438dddbc325ef19

If however we specify the value for the associated type E in f and f2, so that code calling f2() can depend on knowing the type E:

fn f() -> impl X<(), E = ()> {
	XImpl {
		f1: || (),
		f2: |()| (),
		_ghost: PhantomData,
	}
}

fn f2() -> impl X<(), E = ()> {
	f().and(|rb| rb)
}

The compiler seems to forget that any X::F2 implements Fn(Self::E), and stops compiling: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=1e84aea4402bd45da498aace9682b1ad

   Compiling playground v0.0.1 (/playground)
error[E0277]: expected a `std::ops::Fn<((),)>` closure, found `<impl X<()> as X<()>>::F2`
  --> src/lib.rs:51:6
   |
51 |     f().and(|rb| rb)
   |         ^^^ expected an `Fn<((),)>` closure, found `<impl X<()> as X<()>>::F2`
   |
   = help: the trait `std::ops::Fn<((),)>` is not implemented for `<impl X<()> as X<()>>::F2`

error[E0277]: expected a `std::ops::Fn<((),)>` closure, found `<impl X<()> as X<()>>::F2`
  --> src/lib.rs:51:2
   |
12 | pub trait X<T>: Sized {
   | --------------------- required by `X`
...
51 |     f().and(|rb| rb)
   |     ^^^^^^^^^^^^^^^^ expected an `Fn<((),)>` closure, found `<impl X<()> as X<()>>::F2`
   |
   = help: the trait `std::ops::Fn<((),)>` is not implemented for `<impl X<()> as X<()>>::F2`

error[E0277]: expected a `std::ops::Fn<((),)>` closure, found `<impl X<()> as X<()>>::F2`
  --> src/lib.rs:50:12
   |
50 | fn f2() -> impl X<(), E = ()> {
   |            ^^^^^^^^^^^^^^^^^^ expected an `Fn<((),)>` closure, found `<impl X<()> as X<()>>::F2`
51 |     f().and(|rb| rb)
   |     ---------------- this returned value is of type `XImpl<(), (), <impl X<()> as X<()>>::F2, <impl X<()> as X<()>>::F1>`
   |
   = help: the trait `std::ops::Fn<((),)>` is not implemented for `<impl X<()> as X<()>>::F2`
   = note: required because of the requirements on the impl of `X<()>` for `XImpl<(), (), <impl X<()> as X<()>>::F2, <impl X<()> as X<()>>::F1>`
   = note: the return type of a function must have a statically known size

Note that the bug does not happen if F2: Fn() instead of F2: Fn(E):
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=266f91f2ba2f0730148c4dea8c3732a4

It does not happen either if F2: Fn(T) instead of F2: Fn(E):
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=e37b04bfdeec2a1ebddc10a4422931e8

Meta

rustc --version --verbose:

rustc 1.43.1 (8d69840ab 2020-05-04)
binary: rustc
commit-hash: 8d69840ab92ea7f4d323420088dd8c9775f180cd
commit-date: 2020-05-04
host: x86_64-unknown-linux-gnu
release: 1.43.1
LLVM version: 9.0

This also happens on nightly.

[fmease]

Triage: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=1e84aea4402bd45da498aace9682b1ad now compiles successfully. I'm not sure if we have a test for this, we might have one already. It's hard to pinpoint which PR fixed this issue, we might want to bisect this.

[moxian]

Bisects to #73905

full cargo-bisect-rustc output

********************************************************************************
Regression in nightly-2020-10-07
********************************************************************************

fetching https://static.rust-lang.org/dist/2020-10-06/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2020-10-06: 40 B / 40 B [=======================================================] 100.00 % 20.19 KB/s converted 2020-10-06 to a1dfd2490a6cb456b92e469fa550dc217e20ad6d
fetching https://static.rust-lang.org/dist/2020-10-07/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2020-10-07: 40 B / 40 B [======================================================] 100.00 % 205.05 KB/s converted 2020-10-07 to 98edd1fbf8a68977a2a7c1312eb1ebff80515a92
looking for regression commit between 2020-10-06 and 2020-10-07
fetching (via remote github) commits from max(a1dfd2490a6cb456b92e469fa550dc217e20ad6d, 2020-10-04) to 98edd1fbf8a68977a2a7c1312eb1ebff80515a92
ending github query because we found starting sha: a1dfd2490a6cb456b92e469fa550dc217e20ad6d
get_commits_between returning commits, len: 6
  commit[0] 2020-10-05: Auto merge of #77080 - richkadel:llvm-coverage-counters-2, r=tmandry
  commit[1] 2020-10-06: Auto merge of #77606 - JohnTitor:rollup-7rgahdt, r=JohnTitor
  commit[2] 2020-10-06: Auto merge of #77594 - timvermeulen:chain_advance_by, r=scottmcm
  commit[3] 2020-10-06: Auto merge of #73905 - matthewjasper:projection-bounds-2, r=nikomatsakis
  commit[4] 2020-10-06: Auto merge of #76356 - caass:hooks, r=jyn514
  commit[5] 2020-10-06: Auto merge of #77386 - joshtriplett:static-glibc, r=petrochenkov
ERROR: no CI builds available between a1dfd2490a6cb456b92e469fa550dc217e20ad6d and 98edd1fbf8a68977a2a7c1312eb1ebff80515a92 within last 167 days

I see that the PR did add a bunch of tests, but I don't think this specific case was covered (at least, judging by the pre-#73905 error messages of new tests).

@rustbot labels: -E-needs-bisection