Inference disrupted by trait bound that is redundant with a HRTB

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 (https://github.com/Manishearth/rust-clippy/issues/1689).

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

1. 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 (https://github.com/rust-lang/rust-clippy/issues/1689).

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

   David Tolnay at 2020-06-07 01:35:38

2. cc @nikomatsakis @eddyb @arielb1

   Aaron Turon at 2017-05-04 05:10:39

3. This may be a consequence of #21974.

   David Tolnay at 2017-05-04 06:53:36

4. add search keywords: higher ranked lifetime bound

   jethrogb at 2017-09-20 01:01:12

5. 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.

   jethrogb at 2017-09-20 01:06:07