TS not able to infer that T[PropsThatExtend<T, M>] extends M

[Flonk]

Bug Report

🔎 Search Terms

• type inference
• mapped type
• conditional types

🕗 Version & Regression Information

All versions I tested.

⏯ Playground Link

Playground link with relevant code

💻 Code

type PropsThatExtend<T, M> = {
  [K in keyof T]: T[K] extends M ? K : never;
}[keyof T];

const isFour = <T,>(obj: T, prop: PropsThatExtend<T, number>) => {
  return obj[prop] === 4;
}

type Foo = {
  bar: number;
  baz: number | string;
  qux: 2 | 4;
};

/* PropsThatExtend<Foo, number> === "bar" | "qux"; */

isFour({ bar: 1, baz: 2, qux: 4 } as Foo, "qux");

🙁 Actual behavior

Error on line 6:
This condition will always return 'false' since the types 'T[PropsThatExtend<T, number>]' and 'number' have no overlap.

🙂 Expected behavior

Since all members in T[PropsThatExtend<T, number>] extend number the compiler should be able to recognize that the two types being compared overlap.

[jcalz]

Duplicate #30728