Conditional types - Extract works incorrect

[sirian]

TypeScript Version: 2.8.0-rc, 2.9.0-dev.20180327

declare function isFunction<T>(value: T): value is Extract<T, Function>;

declare function toString1(value: object | Function): string ;
declare function toString2(value: Function): string ;

function foo<T>(value: T) {
    if (isFunction(value)) {
        toString1(value); // ok

        toString2(value); // error TS2345: Argument of type 'Extract<T, Function>' is not assignable to parameter of type 'Function'.
                          // Type 'T' is not assignable to type 'Function'.
    }
}

[jcalz]

Is this not a duplicate of #22347?

[ahejlsberg]

Yes, it is pretty much a duplicate. And a workaround similar to the one mentioned in #22347 (comment) can be used:

declare function isFunction<T>(value: T): value is (Extract<T, Function> & Function);

That said, it would be nice if it actually worked, so I'm going to keep this one open as a bug.

[sirian]

It's wrong

declare function isObject<T>(value: T): value is (Extract<T, object> & object);

declare const foo: {key: any} | Record<string, any>;

if (isObject(foo)) {
    console.log(foo.key);
}

error TS2339: Property 'key' does not exist on type '{ key: any; } | Record<string, any>'.

[sirian]

moreover:

[ahejlsberg]

sirian That's an unrelated issue. See #21141. In your example the isObject type guard doesn't actually do anything because both sides of the union are of type object.

[ahejlsberg]

See #21732 regarding the example involving the in operator.

[sirian]

Anders Hejlsberg (@ahejlsberg)

type Bit = 0 | 1;

declare function isBit<T>(value: T): value is Extract<T, Bit> & Bit;

declare function obj(value: object): string ;
declare function bit(value: Bit): string ;

function test1<T>(value: T) {
    if (isBit(value)) {
        obj(value);
        bit(value);
    }
}

function test2<T extends Bit>(value: T) {
    if (isBit(value)) {
        obj(value);
        bit(value);
    }
}

function test3<T extends string>(value: T) {
    if (isBit(value)) {
        obj(value);
        bit(value);
    }
}

[sirian]

Anders Hejlsberg (@ahejlsberg) maybe it should be Extract<T & Bit, Bit>?