NativeCall callback signature syntax?

qsort.p6

#!/usr/bin/env perl6

use NativeCall;

sub qsort(
    CArray[int32] $base,
    size_t $count,
    size_t $size,
    &compare:(
        Pointer[int32] $ap,
        Pointer[int32] $bp
        --> int32
    )
) is native { * }

sub int32-compare (Pointer[int32] $ap, Pointer[int32] $bp --> int32) {
    $ap.deref <=> $bp.deref
}

my $ints = CArray[int32].new(1, 6, 5, 3, 4, 2, 7);
$ints[^ $ints.elems].say;
qsort($ints, $ints.elems, nativesizeof(int32), &int32-compare);
$ints[^ $ints.elems].say;