Coercing higher-ranked function definitions to non-hr function pointers doesn't work

[BenjaminBrienen]

I tried this code:

fn foo() {
    let _: [fn(_); 2] = [x, y];
}
fn x(_: &str) {
    todo!()
}
fn y(_: &str) {
    todo!()
}

I expected to see this happen: compiles

Instead, this happened:

error[E0308]: mismatched types
 --> src/lib.rs:2:29
  |
2 |     let _: [fn(_); 2] = [x, y];
  |                             ^ one type is more general than the other
  |
  = note: expected fn pointer `fn(&_)`
                found fn item `for<'a> fn(&'a _) {y}`

For more information about this error, try `rustc --explain E0308`.

Meta

rustc --version --verbose:

stable 1.89

[BenjaminBrienen]

CC @lcnr

oki, what's happening is that we compute lub(fn_sig(x), fn(_)) which falls back to equality when encountering the binder in the type of x and then fails with a placeholder error

least upper bound of for<'a> fn(&'a str) and fn(_) results in an erorr

https://rust-lang.zulipchat.com/#narrow/stream/144729-t-types/topic/Type.20issue.20with.20fn.20item

[fmease]

Pretty sure this is a dupe of #142970. Prolly not after all.