tsc does not infer "this" correctly in MyClass.prototype.getData syntax

[michael-brade]

TypeScript Version: 2.1.4

Code

Consider this:

class MyClass {
  data: string;
  wrongData: {};
}

interface MyClass {
  getData(): string;
}

MyClass.prototype.getData = function() {
  return this.wrongData; // this.data;
};

Expected behavior:

VS Code should show an error, tsc should refuse to compile it. Hovering this in VS Code should show MyClass, not any.

Actual behavior:

TypeScript 2.1.4 compiles this without complaining. VS Code doesn't show an error. It infers correctly that getData() should return a string, but it thinks this is any.

[mhegazy]

If this type is not specified, it is inferred to be any. consider adding a this declaration to your interface:

interface MyClass {
  getData(this:MyClass): string;
}

[michael-brade]

Well, I guess this is a bit more verbose but it would work as a workaround for now. However, it looks ugly and seems totally unneccessary. MyClass.prototype.xxxx already implies that this is MyClass, doesn't it? So why can't or shouldn't TypeScript infer this correctly?

[mhegazy]

Please see #10288