Compiler can't unify arg-position `impl Trait` into an HRTB

It looks like using arg: impl Trait makes the compiler unable to generalize the entire function-type into HRTB (?), that is:

fn foo(_: &str) {
    //
}

fn bar(_: impl AsRef<str>) {
    //
}

fn zar<T>(_: T) {
    //
}

fn check(_: impl Fn(&str)) {
    //
}

fn main() {
    check(foo); // ok
    check(bar); // err (implementation not general enough)
    check(zar::<&str>); // err (ditto)
}

Using a no-op closure helps:

fn main() {
    check(|x| bar(x));
}

... but is somewhat inconvenient (spotted in the wild at https://github.com/clap-rs/clap/issues/4939).

Note that I'm reporting this is a bug, but I'm not really sure on the category here - it just feels like this code should pass type-checking, but maybe there's some deeper reason why it can't.

Start with the minimal Rust reproducer in the issue and compare the behavior of foo, bar, zar::<&str>, and the closure workaround when passed to check. Trace the compiler's function-type generalization and HRTB handling, then verify that the direct bar and zar cases type-check without regressing the existing examples.