rustc fails to see that trait bound is satisfied

[Freax13]

Code

I tried this code:

use std::marker::PhantomData;

pub struct MyGenericType<T> {
    _marker: PhantomData<*const T>,
}

pub struct MyNonGenericType;

impl<T> From<MyGenericType<T>> for MyNonGenericType {
    fn from(_: MyGenericType<T>) -> Self {
        todo!()
    }
}

pub trait MyTrait {
    const MY_CONSTANT: i32;
}

impl<T> MyTrait for MyGenericType<T>
where
    Self: Into<MyNonGenericType>,
{
    const MY_CONSTANT: i32 = 1;
}

impl<T> MyGenericType<T> {
    const MY_OTHER_CONSTANT: i32 = <MyGenericType<T> as MyTrait>::MY_CONSTANT;
}

I expected to see this happen:
It should compile successfully.

Instead, this happened:
The compiler doesn't realize that Into<MyNonGenericType> is implemented for all MyGenericType<T> and reports an error.

error[E0277]: the trait bound `MyNonGenericType: From<MyGenericType<T>>` is not satisfied
  --> src/lib.rs:27:36
   |
27 |     const MY_OTHER_CONSTANT: i32 = <MyGenericType<T> as MyTrait>::MY_CONSTANT;
   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `From<MyGenericType<T>>` is not implemented for `MyNonGenericType`
   |
   = note: required because of the requirements on the impl of `Into<MyNonGenericType>` for `MyGenericType<T>`
note: required because of the requirements on the impl of `MyTrait` for `MyGenericType<T>`
  --> src/lib.rs:19:9
   |
19 | impl<T> MyTrait for MyGenericType<T>
   |         ^^^^^^^     ^^^^^^^^^^^^^^^^
help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
   |
26 | impl<T> MyGenericType<T> where MyNonGenericType: From<MyGenericType<T>> {
   |                          ++++++++++++++++++++++++++++++++++++++++++++++

For more information about this error, try `rustc --explain E0277`.
error: could not compile `regression` due to previous error

Interestingly enough, removing the where Self: Into<MyNonGenericType>, bound makes this compile again.

cargo-bisect-rustc report

searched nightlies: from nightly-2021-12-01 to nightly-2021-12-25
regressed nightly: nightly-2021-12-14
searched commit range: 6bda5b3...8f117a7
regressed commit: 22f8bde

bisected with cargo-bisect-rustc v0.6.1

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --test-dir=. --start=2021-12-01 --end=2021-12-25 

@rustbot modify labels: +regression-from-stable-to-nightly

[inquisitivecrystal]

CC @fee1-dead, as the author of the relevant PR.

[fee1-dead]

Fixed by #92257, this one is more interesting than the test I added in that PR, as it is stable code (but uses impl const Into<U> for T where U: ~const From<T>), so I think we could add this one as an ui test as well.

[Freax13]

I can confirm that this is fixed on the latest nightly.

[fee1-dead]

It's E-needs-test because someone should add an ui test for this issue

[weirane]

It's E-needs-test because someone should add an ui test for this issue

I'm interested in working on this. Could you tell me where should the test be added? It seems to be somewhere inside src/test/ui.

[inquisitivecrystal]

It's E-needs-test because someone should add an ui test for this issue

I'm interested in working on this. Could you tell me where should the test be added? It seems to be somewhere inside src/test/ui.

I might go with src/test/ui/traits/issue-92292.rs. There isn't really much dark magic to picking which directory to file something under; there are a few directories that aren't really used anymore, but other than that, it's just a matter of picking something that makes sense.

[weirane]

@rustbot claim