ReadonlyKeys

@example

type Props = { readonly foo: string; bar: number };

  // Expect: "foo"
  type Keys = ReadonlyKeys<Props>;

type ReadonlyKeys<T extends object> = [P in keyof T]-?: IfEquals<[Q in P]: T[P], -readonly [Q in P]: T[P], never, P>[keyof T];

§Type Parameters

T extends object

[P in keyof T]-?: IfEquals<[Q in P]: T[P], -readonly [Q in P]: T[P], never, P>[keyof T]