Rustc fails to normalize trait functions that have late-bound lifetimes

[zakarumych]

I tried this code:

trait Foo {
    type Item<'a>: 'a;
    
    fn foo<'a>() -> Self::Item<'a>;
}

struct Bar;

impl Foo for Bar {
    type Item<'a> = ();

    fn foo<'a>() -> () { () }

    // this works
    // fn foo<'a>() -> Self::Item<'a> { () }

    // this works too
    // fn foo<'a>() -> () where (): 'a { () }
}

Playground link

I expected to see this happen: Successful compilation

Instead, this happened: error[[E0195]](https://doc.rust-lang.org/stable/error_codes/E0195.html): lifetime parameters or bounds on method foo do not match the trait declaration

A workaround to this issue is using associated generic type in impl, add trivial where clause (): 'a or
add dummy argument that uses the lifetime

Meta

rustc --version --verbose:

rustc 1.68.0 (2c8cc3432 2023-03-06)
rustc 1.70.0-nightly (22f247c6f 2023-03-13)

[lukas-code]

Looks like unused lifetimes are late-bound, even though they probably shouldn't be: (playground)

fn late<'a>() {}
fn early<'a: 'a>() {}

fn check<'a>() {
    early::<'a>(); // OK
    late::<'a>(); // ERROR: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
}

Edit: Nevermind early vs late is correct for this example.

---

Better minimized example: link

trait Foo {
    fn foo<'a: 'a>(); // early
}

impl Foo for () {
    fn foo<'a>() {} // late, but should be early
}