Associated Type Bounds on Trait Bounds on GAT produce error

[T0mstone]

I hope this is a genuine bug and not just working as intended🙈

I tried this code:

trait Assoc {
	type Type;
}

trait Static: 'static {
	type Same<T>: Assoc<Type: Static>;
}

I expected to see this happen: The code compiles.
Instead, this happened: The code produces the following compile error:

error[E0310]: the associated type `<<Self as Static>::Same<T> as Assoc>::Type` may not live long enough
 --> src/lib.rs:6:28
  |
6 |     type Same<T>: Assoc<Type: Static>;
  |                               ^^^^^^
  |                               |
  |                               the associated type `<<Self as Static>::Same<T> as Assoc>::Type` must be valid for the static lifetime...
  |                               ...so that the type `<<Self as Static>::Same<T> as Assoc>::Type` will meet its required lifetime bounds...
  |
note: ...that is required by this bound
 --> src/lib.rs:5:15
  |
5 | trait Static: 'static {
  |               ^^^^^^^
help: consider adding an explicit lifetime bound
  |
6 |     type Same<T>: Assoc<Type: Static> where <<Self as Static>::Same<T> as Assoc>::Type: 'static;
  |

Note that either removing <T> or replacing it with <T: 'static> makes the error go away.

Meta

rustc --version --verbose:

rustc 1.84.0 (9fc6b4312 2025-01-07)
binary: rustc
commit-hash: 9fc6b43126469e3858e2fe86cafb4f0fd5068869
commit-date: 2025-01-07
host: x86_64-unknown-linux-gnu
release: 1.84.0
LLVM version: 19.1.5

Also persists on latest nightly:

rustc 1.86.0-nightly (f85c6de55 2025-01-26)
binary: rustc
commit-hash: f85c6de55206dbee5ffedfd821df1503a7b92346
commit-date: 2025-01-26
host: x86_64-unknown-linux-gnu
release: 1.86.0-nightly
LLVM version: 19.1.7

[theemathas]

Simplified:

trait Assoc {
    type Type;
}

trait Static: 'static {}

trait Foo {
    type Bar: Assoc<Type: Static>;
}

Error output

error[E0310]: the associated type `<<Self as Foo>::Bar as Assoc>::Type` may not live long enough
 --> src/lib.rs:8:27
  |
8 |     type Bar: Assoc<Type: Static>;
  |                           ^^^^^^
  |                           |
  |                           the associated type `<<Self as Foo>::Bar as Assoc>::Type` must be valid for the static lifetime...
  |                           ...so that the type `<<Self as Foo>::Bar as Assoc>::Type` will meet its required lifetime bounds...
  |
note: ...that is required by this bound
 --> src/lib.rs:5:15
  |
5 | trait Static: 'static {}
  |               ^^^^^^^
help: consider adding an explicit lifetime bound
  |
8 |     type Bar: Assoc<Type: Static> where <<Self as Foo>::Bar as Assoc>::Type: 'static;
  |                                   ++++++++++++++++++++++++++++++++++++++++++++++++++

For more information about this error, try `rustc --explain E0310`.

Also note that changing Assoc<Type: Static> to Assoc<Type: Static + 'static> doesn't fix the problem.

@rustbot labels +A-trait-system +A-associated-items +F-associated_type_bounds