Can't define TAIT inside another function

I tried this code:

#![feature(type_alias_impl_trait)]

fn main() {
    type Opaque = impl Sized;

    #[define_opaque(Opaque)]
    fn make_opaque() -> Opaque {
        1i32
    }
}

I expected the code to compile. But instead I got this error:

error: item does not constrain `Opaque::{opaque#0}`
 --> src/main.rs:3:4
  |
3 | fn main() {
  |    ^^^^
  |
  = note: consider removing `#[define_opaque]` or adding an empty `#[define_opaque()]`
note: this opaque type is supposed to be constrained
 --> src/main.rs:4:19
  |
4 |     type Opaque = impl Sized;
  |                   ^^^^^^^^^^

@rustbot labels +F-type_alias_impl_trait

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Reproduces on the playground with version 1.91.0-nightly (2025-08-11 1ebbd87a62ce96a72b22)

Start with the reproducer in src/main.rs and run it on the reported nightly version or the playground to confirm the nested type alias impl trait failure. Trace the compiler handling of #[define_opaque] for the local Opaque alias; done means the example compiles without the constraint error and coverage exists for this case.