Strange behavior with duplicated supertraits and lifetimes.

I'm not sure if this is a bug or not.

The following code does not compile:

trait Super<'a, T> {}
trait Sub<'a, 'b>: Super<'a, i32> + Super<'b, i32> {}

Error output:

error[E0283]: type annotations needed: cannot satisfy `Self: Super<'a, i32>`
 --> src/lib.rs:2:20
  |
2 | trait Sub<'a, 'b>: Super<'a, i32> + Super<'b, i32> {}
  |                    ^^^^^^^^^^^^^^
  |
note: multiple `impl`s or `where` clauses satisfying `Self: Super<'a, i32>` found
 --> src/lib.rs:2:20
  |
2 | trait Sub<'a, 'b>: Super<'a, i32> + Super<'b, i32> {}
  |                    ^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0283`.

The following code doesn't compile:

trait Super<'a, T> {}
trait Sub<'a, 'b, T, U>: Super<'a, T> + Super<'b, U> {}
trait Lol<'a, 'b>: Sub<'a, 'b, i32, i32> {}

Error output:

error[E0283]: type annotations needed: cannot satisfy `Self: Super<'a, i32>`
 --> src/lib.rs:3:20
  |
3 | trait Lol<'a, 'b>: Sub<'a, 'b, i32, i32> {}
  |                    ^^^^^^^^^^^^^^^^^^^^^
  |
note: multiple `impl`s or `where` clauses satisfying `Self: Super<'a, i32>` found
 --> src/lib.rs:3:20
  |
3 | trait Lol<'a, 'b>: Sub<'a, 'b, i32, i32> {}
  |                    ^^^^^^^^^^^^^^^^^^^^^
note: required by a bound in `Sub`
 --> src/lib.rs:2:26
  |
2 | trait Sub<'a, 'b, T, U>: Super<'a, T> + Super<'b, U> {}
  |                          ^^^^^^^^^^^^ required by this bound in `Sub`

For more information about this error, try `rustc --explain E0283`.

The following code compiles

trait Super<'a, T> {}
trait Sub<'a, 'b, T, U>: Super<'a, T> + Super<'b, U> {}
trait Lol: for<'a, 'b> Sub<'a, 'b, i32, i32> {}
fn wut(_: &dyn Lol) {}

The following code also compiles:

trait Super<'a, T> {}
trait Sub<'a, 'b, T, U>: Super<'a, T> + Super<'b, U> {}
trait Lol: Sub<'static, 'static, i32, i32> {}
fn wut(_: &dyn Lol) {}

The following code does NOT compile:

trait Super<'a, T> {}
trait Sub<'a, 'b, T, U>: Super<'a, T> + Super<'b, U> {}
trait Lol: for<'a> Sub<'a, 'static, i32, i32> {}
fn wut(_: &dyn Lol) {}

Error output:

error[E0283]: type annotations needed: cannot satisfy `Self: Super<'static, i32>`
 --> src/lib.rs:3:12
  |
3 | trait Lol: for<'a> Sub<'a, 'static, i32, i32> {}
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: multiple `impl`s or `where` clauses satisfying `Self: Super<'static, i32>` found
 --> src/lib.rs:3:12
  |
3 | trait Lol: for<'a> Sub<'a, 'static, i32, i32> {}
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: required by a bound in `Sub`
 --> src/lib.rs:2:41
  |
2 | trait Sub<'a, 'b, T, U>: Super<'a, T> + Super<'b, U> {}
  |                                         ^^^^^^^^^^^^ required by this bound in `Sub`

For more information about this error, try `rustc --explain E0283`.

I don't understand what the compiler is doing.

Discovered while investigating #154662.

Meta

Reproducible on the playground with rust 1.96.0.

No repository file or test is identified. Start by running the listed Rust 1.96.0 playground reproductions and compare the successful and failing lifetime cases, then review the related investigation in issue #154662. Done means explaining or correcting the duplicated-supertrait and lifetime behavior with a regression test covering the examples.