Type Merging between extends and intersection

[kitsonk]

TypeScript Version:

1.8.10

Code

interface Foo {
    on(type: 'foo', listener: (event: MouseEvent) => boolean): boolean;
    on(type: string, listener: (event: Event) => boolean): boolean;
}

interface Bar {
    on(type: 'bar', listener: (event: MSGestureEvent) => boolean): boolean;
    on(type: string, listener: (event: Event) => boolean): boolean;
}

interface FooBarA extends Foo, Bar {} // Error: not correctly extended

interface FooBarB extends Foo, Bar {
    on(type: 'foo', listener: (event: MouseEvent) => boolean): boolean;
    on(type: 'bar', listener: (event: MSGestureEvent) => boolean): boolean;
    on(type: string, listener: (event: Event) => boolean): boolean;
}

type FooBarC = Foo & Bar;

let foobar: FooBarC;

foobar.on('foo', (event) => { return false; });

Expected behavior:

That FooBarA would work instead of throwing an error.

Actual behavior:

FooBarA complains Interface 'FooBarA' cannot simultaneously extend types 'Foo' and 'Bar'. Named property 'on' of types 'Foo' and 'Bar' are not identical.

While that is true in the strictest sense, the "base" override matches, which means in theory the string literal types could be merged and the error should only occur if there is a conflict between a specific string literal type.

Using the intersection type works perfectly fine and the resulting type mirrors the runtime behaviour. Using the "reimplement all methods" (FooBarB) gets really tedious really quickly when you are trying to do something like model event listeners like the above.

[RyanCavanaugh]

Proposed fix: When we consider signatures, after flattening the list of all signatures, we should then stable-sort, placing signatures with the more string literal types before those with fewer.

[kitsonk]

I am going to try to tackle this.

[rjamesnw]

I just ran into something similar also.

class A {
    private value?: any;
}

interface IA extends A { } 

class B {
    private value?: any;
}

interface IB extends B, IA { } 

(https://goo.gl/Yo4LJA)

Interface 'IB' cannot simultaneously extend types 'B' and 'IA'.
Named property 'value' of types 'B' and 'IA' are not identical.

Actually, I was surprised it even cares to pull in private members to a public interface.

[Kingwl]

ping @RyanCavanaugh
i don't understand why sort the signatures can solve this issue....
Can you explain it for me? 😄

[HolgerJeromin]

My usecase was this:

class MyClass { };
export interface IOptionsBase {
}
export interface IOptions extends IOptionsBase {
    myCb?: (this: Window, data: boolean) => void;
}
export interface IOptionsEx extends IOptionsBase {
    myCb?: (this: MyClass, data: number) => void;
}

// should get:       myCb?: (this: Window|MyClass, data: number) => void;
export interface ICtrlMetaData extends IOptions, IOptionsEx {
}