impls with TAIT reject valid unconstraining lifetimes

[aliemjay]

I tried this code:

#![feature(impl_trait_in_assoc_type)]

trait Callable {
    type Out;
    fn call() -> Self::Out;
}

impl<'a> Callable for () {
    type Out = impl Sized;
    fn call() -> Self::Out {}
}

(Playground)

I expected this to compile fine since it is accepted without TAIT.

Instead, code was rejected because lifetime 'a is non-constraining.

error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
 --> src/main.rs:8:6
  |
8 | impl<'a> Callable for () {
  |      ^^ unconstrained lifetime parameter

error[E0792]: expected generic lifetime parameter, found `'_`
  --> src/main.rs:10:28
   |
8  | impl<'a> Callable for () {
   |      -- this generic parameter must be used with a generic lifetime parameter
9  |     type Out = impl Sized;
10 |     fn call() -> Self::Out {}
   |                            ^^

[aliemjay]

@rustbot label F-type_alias_impl_trait T-compiler

[oli-obk]

fwiw, the limitation can be worked around by moving the type alias impl trait into a free type alias, which just has the generics that you want to bind.

#![feature(type_alias_impl_trait)]

trait Callable {
    type Out;
    fn call() -> Self::Out;
}

type Any = impl std::any::Any;

impl<'a> Callable for () {
    type Out = Any;
    fn call() -> Self::Out {}
}

[oli-obk]

Triage: not a blocker for stabilization, as workarounds exist. Definitely a bug we should fix though.