Polymorphic this behavior after method extraction

[Guerric-P]

Bug Report

After extracting a method from a class prototype (which becomes a plain function), the polymorphic this still behaves like the function is attached to the class prototype.

🔎 Search Terms

polymorphic this

🕗 Version & Regression Information

I saw this happening in the latest current version (4.4.2) and I don't know since when the bug exists.

⏯ Playground Link

Playground link with relevant code

💻 Code

class Foo {
  axisControl: string = "string";
  directionControl: string = "string";

  filteredAxis: string[] = ["string"];
  filteredDirection: string[] = ["string"];

  test(filteredName: `filtered${string}` & keyof this) {
    return this[filteredName];
  }
}

const foo = new Foo;
const result = foo.test("filteredAxis"); // Type resolves here: string[]

const test = Foo.prototype.test;
const wrong = test("filteredAxis"); // It also resolves here to string[]

const bound = test.bind(undefined);
const wrong2 = bound("filteredAxis"); // It also resolves here to string[]

const wrong3 = Foo.prototype.test.apply(undefined, ["filteredAxis"]); // It also resolves here to string[]

const wrong4 = Foo.prototype.test.call(undefined, "filteredAxis"); // It also resolves here to string[]

🙁 Actual behavior

The wrong, wrong2, wrong3, wrong4 variables types resolves to string[]

🙂 Expected behavior

Compilation errors to prevent the TypeError: Cannot read properties of undefined runtime errors

[andrewbranch]

Note that you can opt into enforcing this by explicitly annotating a this type for the method. I know I’ve heard some team members express that they wish we did this automatically for class methods, but by now it would probably be too big a breaking change. I feel confident there must be an issue discussing this as a suggestion, but I’m having trouble finding it.

[andrewbranch]

Just had to be extra clever with GitHub search. Duplicate of #7968.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.