Inference disrupted by trait bound that is redundant with a HRTB

[dtolnay]

This compiles:

trait Trait<'a> {}

struct S<T>(T);

impl<'a, T> Trait<'a> for S<T> where T: for<'b> Trait<'b> {}

fn main() {}

This following does not. It seems like the impl already has T: for<'b> Trait<'b> so adding T: Trait<'a> should not affect the behavior.

trait Trait<'a> {}

struct S<T>(T);

// Added T: Trait<'a>
impl<'a, T> Trait<'a> for S<T> where T: for<'b> Trait<'b>, T: Trait<'a> {}

fn main() {}

error[E0283]: type annotations required: cannot resolve `T: Trait<'a>`
 --> src/main.rs:6:1
  |
6 | impl<'a, T> Trait<'a> for S<T> where T: for<'b> Trait<'b>, T: Trait<'a> {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: required by `Trait`

This currently affects the following and many people have run into it. The workaround is to remove the bound (rust-lang/rust-clippy#1689).

#[derive(Deserialize)]
struct S<T: serde::de::DeserializeOwned>(T);

[aturon]

cc @nikomatsakis @eddyb @arielb1

[dtolnay]

This may be a consequence of #21974.

[jethrogb]

add search keywords: higher ranked lifetime bound

[jethrogb]

If this is unlikely to be fixed soon, can we at least make the error message better? It's unclear what type annotations could be added and where, and as @dtolnay writes, the fix is actually to be less restrictive.