Type Merging between extends and intersection

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.

Inizia con la riproduzione di TypeScript 1.8.10 nell'issue e confronta il modo in cui FooBarA estende Foo e Bar con l'intersezione equivalente Foo & Bar. Il lavoro è completo quando gli overload compatibili vengono uniti per FooBarA, mentre gli overload specifici di string-literal realmente in conflitto continuano a produrre un errore.