E0794 has a seemingly incorrect definition of "late bound lifetime parameter"

[lolbinarycat]

Location

https://doc.rust-lang.org/stable/error_codes/E0794.html

Summary

The docs say that a lifetime parameter must appear in an argument to be considered "late bound", but the following snippet emits E0794:

fn foo<'a>() {
    
}

fn bar() {
    let _ = foo::<'static>();
}

[Jules-Bertholet]

IIUC it should be “appears in an argument type, or does not appear in the return type”.

[QuineDot]

I agree the E0794 definition is incomplete.

Here are some conditions for being late bound. However, note that the "constrained" section is (AFAICT) talking about lifetimes that appear in the output type, so doesn't apply to the OP.

// I.e. this lifetime is still late bound
fn unused_param<'a>() {}

Also note that "constrained" is not as simple as "appears in".

trait Trait {
    type Gat<'a>;
    // Lifetimes in projections do not constrain
    fn both_are_early_bound<'s:'s, 'a>(&'s self, _: Self::Gat<'a>) -> Self::Gat<'a>;
    // Unconstrained but not in output type so still late bound
    // (similar to `unused_param`)             vv
    fn both_are_late_bound(&self, _: Self::Gat<'_>);
}

And "appear in the output type" includes elided lifetimes, including those implicitly captured by opaque types.

pub trait Trait {
    type Gat<'a>;
    fn first_lt_late_bound_second_lt_early_bound(&self, _: Self::Gat<'_>) -> impl Sized;
}

Which leads to confusing errors when eschewing aliases or using refinement: see #87803, #109476, #135350.