function strictIsDog<T extends Dog extends T ? unknown : never>( // like <T super Dog> candidate: Dog | T // if Dog extends T then Dog | T is T ): candidate is Dog { // compiler recognizes that Dog | T can narrow to T return "bark" in candidate; } if (strictIsDog(animal)) {} // okay if (strictIsDog(dog)) {} // okay if (strictIsDog(mixed)) {} // okay if (strictIsDog(cat)) {} // error! // ~~~ <-- Cat is not assignable to Dog

Adding to the accepted answer, if you happen to need to use a type guard against a mixin, you'll get this error too, since the is operator doesn't behave as an implements would.

const isValidMethodForHandler = <T extends { [i: string]: any }>(handler: T) => ( method: string ): method is Extract<keyof T, string> => Object.keys(handler).indexOf(method) !== -1;