`const_trait_impl`s cannot be utilized with a `trait_alias`

[AZMCode]

const_trait_impl cannot utilize const traits when satisfying const bounds using a trait alias trait_alias

#![feature(trait_alias)]
#![feature(const_trait_impl)]

#[const_trait]
trait Default {
    fn default() -> Self;
}

#[const_trait]
trait TraitAlias = ~const Default + Sized;

#[const_trait]
trait OwnTrait: ~const Default + Sized {
    const INTERESTING_CONSTANT: Self;
}

impl<T: ~const TraitAlias + Sized> const OwnTrait for T {
    const INTERESTING_CONSTANT: Self = T::default();
}

Link to Playground

There is not way to specify const-ness using trait bounds. The compile error does specify adding a #[const_trait] attribute, but this is not possible for trait aliases;

rustc --version --verbose:

rustc 1.68.0-nightly (d6f99e535 2023-01-02)
binary: rustc
commit-hash: d6f99e535a301a421dfee52a7c25bb4bdf420344
commit-date: 2023-01-02
host: x86_64-unknown-linux-gnu
release: 1.68.0-nightly
LLVM version: 15.0.6

[fmease]

Fixed in #144291.

This is how the code above should be changed to to work:

#![feature(trait_alias, const_trait_impl)]

const trait Default { fn default() -> Self; }

const trait TraitAlias = [const] Default + Sized;

const trait OwnTrait: [const] Default + Sized {
    const INTERESTING_CONSTANT: Self;
}

impl<T: const TraitAlias> const OwnTrait for T {
    const INTERESTING_CONSTANT: Self = T::default();
}

Note that you have to change the conditionally const bound in the trait impl, ~const TraitAlias, to an unconditionally const bound, const TraitAlias. That has nothing to do with trait aliases, that's simply necessary because the associated constant is obviously always const regardless of whether OwnTrait is used in a const or non-const context.

---

As an aide, depending on your requirements you could even simplify this by applying the following changes:

- const trait TraitAlias = [const] Default + Sized;
+ const trait TraitAlias = const Default + Sized;

- impl<T: const TraitAlias> const OwnTrait for T {
+ impl<T: TraitAlias> const OwnTrait for T {

However, it's likely too restricted in your actual code base. I'm just throwing it out there for illustrative / pedagogical purposes.