5.23.1 unsafe cast

Unsafe casts are useful to subvert the type system. The compiler types expr as usual and then wraps it in a monomorph. This allows the expression to be assigned to anything.

Unsafe casts do not introduce any dynamic types, as the following example shows:

class Main {
  public static function main() {
    var i = 1;
    $type(i); // Int
    var s = cast i;
    $type(s); // Unknown<0>
    $type(s); // String

Variable i is typed as Int and then assigned to variable s using the unsafe cast cast i. This causes s to be of an unknown type, a monomorph. Following the usual rules of unification, it can then be bound to any type, such as String in this example.

These casts are called "unsafe" because the runtime behavior for invalid casts is not defined. While most dynamic targets are likely to work, it might lead to undefined errors on static targets.

Unsafe casts have little to no runtime overhead.