Typescript: Intersection type containing a constructor function type should count as one

[andy-ms]

TypeScript Version: nightly

Code

class A {}
const a: (typeof A) & {} = A;
class B extends a {}

Expected behavior:

No error.

Actual behavior:

src/a.ts(3,17): error TS2507: Type 'typeof A & {}' is not a constructor function type

[ahejlsberg]

Is there a real world scenario that needs this?

[andy-ms]

I was playing around with mixins, which involve intersecting class types. It would take more than just this to make those work, though!
The following might (or might not) be possible:

    interface MixinStatics {
        mixinStatic(): void;
    }
    interface MixinInstance {
        mixinInstance(): void;
    }

    function mixin<Instance, Cls extends { new(): Instance }>(cls: Cls): { new(): Instance & MixinInstance } & MixinStatics & Cls {
        return <any> class extends cls {
            static mixinStatic() {}
            mixinInstance() {}
        }
    }

    class Super {}
    // Type annotation hopefully not necessary
    class C extends mixin<Super, typeof Super>(Super) { }

    C.mixinStatic();
    new C().mixinInstance();

[HerringtonDarkholme]

One real world scenario is mixin http://justinfagnani.com/2015/12/21/real-mixins-with-javascript-classes/. If counting intersection type with construct signature would at least partially solve the long awaited mixin support in TypeScript.

[Draccoz]

Just got here with exactly same issue. Mixins are very important, so far possible with ES6, so would be great if we could actually do that in TS.

[imcotton]

Seems quite handy in ES6

const Storage = Sup => class extends Sup {
    save(database) {  }
};
const Validation = Sup => class extends Sup {
    validate(schema) {  }
};

class Person {  }
class Employee extends Storage(Validation(Person)) {  }

http://exploringjs.com/es6/ch_classes.html#_simple-mixins

[andy-ms]

Duplicate of #4890?