Suggestion: typechecking of 'this' context

[RyanCavanaugh]

A common error in TypeScript is to lose the this context, especially in class situations:

class MyClass {
    x = 4;
    printX() { console.log(this.x); }
}
var m = new MyClass();
window.setInterval(m.printX, 1000); // Prints 'undefined', not '4'

The compiler should detect when this happens and issue an error:

var m = new MyClass();
// Error, `this` context might be lost
window.setInterval(m.printX, 1000);

We need a proposal here that determines why this would be an error, in a way that is not burdensome. For example, we do want it to be valid to call e.g. function#bind on m.printX, and it should probably valid to write something like if(m.printX) { m.printX() }.

This gets even trickier when we look at the members of window -- some of them can be invoked with any this, but others cannot. It should be possible to describe these differences.

[metaweta]

A solution to this problem would need to distinguish between methods that should only be used on instances of MyClass and methods that can be used on any object satisfying an interface that MyClass satisfies. For example, all the methods on Array.prototype are "generic" in the sense that they can be applied to any "array-like" object, i.e. one with a length property and numeric properties, not just to instances of Array.

[mhegazy]

This is now covered by #3694. PR is in #6739