TryFrom trait bound on trait definition complains about missing From impl

[adamlesinski]

I tried this code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=a9fcf72c4bbeb63baf3dffb724d37852

I expected it to compile successfully.

Instead, it fails to compile with the error:

error[E0277]: the trait bound `&'a <C as Container>::Item: From<&'a Wrapper>` is not satisfied

I do not expect that a From implementation is needed when implementing TryFrom.

If I remove the trait bound from the trait Container, and place it on the function foo instead, it works as expected: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=a4bdca2f01cd2a33f85d18e5ebdb5181

Also, if the trait bound is changed to not include lifetimes or references, this works as well: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=a2e8dbad7838e71b6c0b8a42fca57509

The issue happens on both Beta and Nightly.

[SkiFire13]

This is a duplicate of #20671

It complains about From because when looking for candidate impls for for<'a> &'a <C as Container>::Item: TryFrom<&'a Wrapper> it found impl<T, U> TryFrom<U> for T where U: Into<T>, then impl<T, U> Into<U> for T where U: From<T>, which leads to for<'a> &'a <C as Container>::Item: From<&'a Wrapper>.

[adamlesinski]

I see, but impl<T, U> TryFrom<U> for T where U: Into<T> shouldn't be a candidate, given that none of the bounds are satisfied. It should see my local implementation of TryFrom right?

[SkiFire13]

It sees your local TryFrom implementation and correctly accepts it, however the problem isn't on the call foo::<WrappedContainer>() (you can even remove that call and the error will still be there, playground example), the problem is on the definition of foo itself (fn foo<C: Container>() {}), in particular that C: Container. There the trait solver assumes that C implements the trait Container, however it doesn't assume it satisfies all the where clauses, in particular those that aren't directly on Self or on associated items (due to the bug I mentioned in the previous message), but it still tries to check that they're satisfied, and this obviously fails because they aren't valid for all C. This is why the error message mentions for<'a> &'a <C as Container>::Item: TryFrom<&'a Wrapper> with the generic C and not with your type substituted.

ps: I think the way rustc works is that impl<T, U> TryFrom<U> for T where U: Into<T> becomes a candidate because it may apply without considering the bounds. Then the bounds are considered and they may lead to searching for other impls or to a dead end that removes it from the candidates. However i don't know why it's still there in the error message, I would expect it to show the original predicate it was trying to satisfy (for<'a> &'a <C as Container>::Item: TryFrom<&'a Wrapper>).

[Dylan-DPC]

Current complete error:

error[[E0277]](https://doc.rust-lang.org/stable/error_codes/E0277.html): the trait bound `for<'a> &'a <C as Container>::Item: From<&'a Wrapper>` is not satisfied
  --> src/main.rs:30:11
   |
30 | fn foo<C: Container>() {}
   |           ^^^^^^^^^ the trait `for<'a> From<&'a Wrapper>` is not implemented for `&'a <C as Container>::Item`
   |
   = note: required for `&'a Wrapper` to implement `for<'a> Into<&'a <C as Container>::Item>`
   = note: required for `&'a <C as Container>::Item` to implement `for<'a> TryFrom<&'a Wrapper>`
note: required by a bound in `Container`