Optional Chaining does not work with generic this parameter

[osyrisrblx]

Bug Report

🔎 Search Terms

optional chain, generic, this, method

🕗 Version & Regression Information

Bug started in TypeScript 3.7.

• I was unable to test this on prior versions because optional chaining did not exist before 3.7

⏯ Playground Link

Playground link with relevant code

💻 Code

interface X {
    foo<T>(this: T, arg: keyof T): void;
}

interface Y extends X {
    a: number;
    b: string;
}

declare const value: Y | undefined;

// this works
if (value) {
    value?.foo("a");
}

// this doesn't
value?.foo("a"); // Argument of type 'string' is not assignable to parameter of type 'never'.

🙁 Actual behavior

Generic T is not resolved. keyof T results in never.

🙂 Expected behavior

Generic T should resolve to Y so that keyof T resolves to "a" | "b" | "foo".

[RyanCavanaugh]

Repros without inheritance

interface Y {
    foo<T>(this: T, arg: keyof T): void;
    a: number;
    b: string;
}

declare const value: Y | undefined;

// this works
if (value) {
    value?.foo("a");
}

// this doesn't
value?.foo("a");