Type mismatch when using constant with type_alias_impl_trait

[joshlf]

I tried this code (playground):

#![feature(type_alias_impl_trait)]

pub type IntoUnit = impl Into<()>;

pub const _: IntoUnit = ();

pub fn infer() -> IntoUnit {
    ()
}

I expected to see this happen: Successful compilation

Instead, this happened: While infer compiled successfully, the anonymous constant did not:

error[E0308]: mismatched types
 --> src/lib.rs:5:25
  |
3 | pub type IntoUnit = impl Into<()>;
  |                     ------------- the expected opaque type
4 | 
5 | pub const _: IntoUnit = ();
  |                         ^^ expected opaque type, found `()`
  |
  = note: expected opaque type `impl Into<()>`
               found unit type `()`

Meta

Rust playground version: 1.49.0-nightly (2020-10-14 e160e5c)

cc @jswrenn

[jonas-schievink]

cc #77898, a similar issue

[jackh726]

This actually works now, even with min_type_alias_impl_trait (though it shouldn't)

[nikomatsakis]

We should decide whether const X: Foo is a "defining use" for TAIT. I think my rule is roughly "if impl Trait can be used there to mean "existential type", it is a defining use". Presently this does not include const, so this should not be accepted by min_type_alias_impl_trait.

However, I would eventually expect this code to compile, yes.

[oli-obk]

This works now, and I expect this to keep working, as there's no implementation reason to forbid it and no language reason that I know of. Theoretically all functions could just be constants that return an impl Fn() type 😆

[nikomatsakis]

Huzzah! @oli-obk is awesome. Closing.