[Bug] Assigning an interface with additional properties to an interface with fewer properties fails

[Luna-Klatzer]

Is there an existing issue for this?

• I have searched the existing issues

I am following the documentation's guide

• I have read the documentation

This issue exists in the latest version

• I am using the latest version

Current Behavior

Assigning an interface with additional properties to an interface with fewer properties fails. While this is common in some programming languages, this also goes against the duck typing principle that as long as all properties of an interface are present it is compatible, even if additional unrelated properties are present.

Expected Behavior

The assignment should work without error.

Steps To Reproduce

1. Define one interface X with one property x: num
2. Define one interface Y with the properties x: num and y: num (Y "extends" X and is compatible with it)
3. Assign a value that is typed as Y to a variable with type X.

Example:

interface X {
	x: num;
}

interface Y {
	x: num;
	y: num;
}

var y: Y = {
	y: 1,
	x: 2,
};
var x: X = y;

Environment

• Kipper: 0.12.0
• Environment: Firefox Nightly 133.0b6
• Operating System: Arch Linux