Cast to class or interface

[FranklinWhale]

Describe the bug In PXT 4.4, cast to class or interface is not supported. In PXT 5.5, no error is displayed on the JavaScript editor but the code does not run.

To Reproduce Steps to reproduce the behavior:

1. Go to https://makecode.microbit.org/beta
2. Switch to JavaScript
3. Paste the code below to the editor

   interface Length {
   length: number;
   }
   function showLength(x: Length) {
   basic.showNumber(x.length);
   }
   showLength([]);
   showLength("X");

4. See error

Expected behavior Either the cast is disallowed with error or it works successfully.

[riknoll]

This is the expected behavior, but we should probably have a better error in the simulator. The error thrown on hardware is 984 (invalid cast).

See https://makecode.com/js/errorcodes

[FranklinWhale]

The issue here is that member access to string, array and object are handled differently in PXT runtime and, when the type is an interface, it is handled as if it is an object and thus passing string and array into the function causes runtime error.

To make the function work at runtime, the showLength function can be written as follows:

function showLength(x: Length) {
    basic.showNumber(typeof x === "string" ? x.length : Array.isArray(x) ? (x as any[]).length : x.length)
}

Perhaps the error message may mention that an explict type assertion may be required.