Function item subtyping can change behavior

I'm not sure if this is a bug or not.

In the following code, F2 is a subtype of F1. It seems that this causes the type of the F2::method function item to be a subtype of the F1::method function item. This implies that function items that are subtypes of each other can have different behavior. This is rather strange.

type F1 = fn(&'static ());
type F2 = for<'a> fn(&'a ());

trait Trait: Sized {
    fn method() -> Option<Self>;
}

impl Trait for F1 {
    fn method() -> Option<Self> {
        println!("F1");
        None
    }
}

#[expect(coherence_leak_check)]
impl Trait for F2 {
    fn method() -> Option<Self> {
        println!("F2");
        None
    }
}

fn main() {
    let mut x = F1::method;
    x();
    let y = F2::method;
    y();
    x = y;
    x();
}

The above code prints:

F1
F2
F1

cc @lcnr

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Reproducible on the playground with version 1.97.0-nightly (2026-04-28 37d85e592f9ae5f20f7d)