fn foo<'a>(a: &'a u32) -> &'a u32 {
 a
}
fn bar<T>(a: fn(&u32) -> T) -> T {
 a(&10)
}
fn main() {
 let higher_ranked_fn_ptr = foo as for<'a> fn(&'a u32) -> &'a u32;
 let references_bound_vars = bar(higher_ranked_fn_ptr);
}

error[E0308]: mismatched types
 --> src/main.rs:10:37
 |
10 | let references_bound_vars = bar(higher_ranked_fn_ptr);
 | --- ^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
 | |
 | arguments to this function are incorrect
 |
 = note: expected fn pointer `for<'a> fn(&'a _) -> _`
 found fn pointer `for<'a> fn(&'a _) -> &'a u32`
note: function defined here
 --> src/main.rs:4:4
 |
4 | fn bar<T>(a: fn(&u32) -> T) -> T {
 | ^^^ ----------------
For more information about this error, try `rustc --explain E0308`.
error: could not compile `foo` (bin "foo") due to 1 previous error