min_specialization does not equate T: ~const Trait with T: Trait

[BGR360]

Given the following code (playground):

#![feature(rustc_attrs)]
#![feature(min_specialization)]
#![feature(const_trait_impl)]

trait DefaultBound {}

#[rustc_specialization_trait]
trait SpecializedBound {}

trait Foo {}

impl<T> const Foo for T
where
    T: ~const DefaultBound
{}

impl<T> Foo for T
where
    T: DefaultBound + SpecializedBound
{}

The following error is produced:

error: cannot specialize on trait `DefaultBound`
  --> src/main.rs:17:1
   |
17 | / impl<T> Foo for T
18 | | where
19 | |     T: DefaultBound + SpecializedBound
20 | | {}
   | |__^

This indicates that rustc is not equating the T: ~const DefaultBound in the default impl with the T: DefaultBound in the specialized impl. Rather, it thinks it's a newly-introduced trait bound, and thus it says "cannot specialize on it" since it's not annotated as rustc_specialization_trait.

In my opinion, the T: ~const DefaultBound and T: DefaultBound should be considered equivalent in the context of specialization, and the above code should compile just as the following code does (playground):

#![feature(rustc_attrs)]
#![feature(min_specialization)]

trait DefaultBound {}

#[rustc_specialization_trait]
trait SpecializedBound {}

trait Foo {}

impl<T> Foo for T
where
    T: DefaultBound
{}

impl<T> Foo for T
where
    T: DefaultBound + SpecializedBound
{}

See also #95186

@rustbot label +F-const_trait_impl +F-specialization +A-specialization +A-traits +requires-nightly

[BGR360]

@rustbot claim