Not all function items implement `Fn*` traits

[xmh0511]

https://doc.rust-lang.org/reference/types/function-item.html#r-type.fn-item.traits says:

All function items implement Fn, FnMut, FnOnce, Copy, Clone, Send, and Sync.

Consider this example:

fn check<F: Fn()>(_: F) {}
extern "C" fn foo() {}
fn main() {
    check(foo);
}

The compiler reports an error:

the trait `Fn()` is not implemented for fn item `extern "C" fn() {foo}`