class Foo {
    foo(a: number): 1;
    foo(a: string): 2;
    foo(a: boolean): 3;
    foo(a: boolean | number | string): 1 | 2 | 3 {
        if (typeof a === 'number') return 1;
        if (typeof a === 'string') return 2;
        return 3;
    }
}

interface IFoo {
    foo<T extends 1>(a: number): T;
    foo<T extends 2>(a: string): T;
    foo<T extends 3>(a: boolean): T;
}

type F = {
    <T extends 1>(a: number): T;
    <T extends 2>(a: string): T;
    <T extends 3>(a: boolean): T;
}

class Bar2 implements IFoo {
    foo(a) { // error here: ts 7006. expected to be number | string | boolean
        if (typeof a === 'number') return 1;
        if (typeof a === 'string') return 2;
        return 3;
    }
}

class Bar implements IFoo {
    foo(a: number | string | boolean) {
        if (typeof a === 'number') return 1;
        else if (typeof a === 'string') return 2;
        else return 3;
    }
}

class Baz {
    foo: F = (a: any) => {
        if (typeof a === 'number') return 1;
        if (typeof a === 'string') return 2;
        return 3;
    }
}

new Foo().foo(1); // infer as 1
new Baz().foo('1'); // infer as 2;
new Bar().foo(true) // infer as 1 | 2 | 3, expected to be 3;