non-defining opaque type use: bogus "generic lifetime used twice"

[aliemjay]

I expect this to compile:

#![feature(type_alias_impl_trait)]
type Opaque<'lt> = impl Sized + 'lt;
fn test<'a>(
    arg: impl Iterator<Item = &'a u8>,
) -> impl Iterator<Item = Opaque<'a>> {
    arg
}

Instead, it is an error in 1.73.0-nightly (2023-08-16 07438b0):

error: non-defining opaque type use in defining scope
 --> src/lib.rs:5:6
  |
5 | ) -> impl Iterator<Item = Opaque<'a>> {
  |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ generic argument `'a` used twice
  |
note: for this opaque type
 --> src/lib.rs:5:6
  |
5 | ) -> impl Iterator<Item = Opaque<'a>> {
  |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: item constrains opaque type that is not in its signature
 --> src/lib.rs:5:6
  |
5 | ) -> impl Iterator<Item = Opaque<'a>> {
  |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this item must mention the opaque type in its signature in order to be able to register hidden types
note: this item must mention the opaque type in its signature in order to be able to register hidden types
 --> src/lib.rs:3:4
  |
3 | fn test<'a>(
  |    ^^^^

[compiler-errors]

First error should be fixed by ignoring the bivariant RPIT params. The second error should go away if we fix the first one.

[wenym1]

From my practice, the rust compiler has problem in nested impl Trait with lifetime. In the code above, the code is similar to impl Iterator<impl Sized>, which is a nested impl Trait.

We can try the following way to avoid nested-ly deducing the concrete type of impl Trait.

#![feature(type_alias_impl_trait)]

type Opaque<'lt> = impl Sized + 'lt;
fn test<'a>(
    arg: impl Iterator<Item = &'a u8>,
) -> impl Iterator<Item = Opaque<'a>> {
    arg.map(identity)
}

fn identity<'a>(input: &'a u8) -> Opaque<'a> {
    input
}

The basic idea is to deduce the concrete type of Opaque in a separate function, so that the compiler would not have to deduce the two nested impl Trait in the same place.