Mixin with union as base class

TypeScript Version: 4.1.0-dev.20201015

Search Terms:

Code

abstract class Base {
    abstract p1: number;
}

class Derived1 extends Base {
    p1 = 1;
    p2 = 2;
}

class Derived2 extends Base {
    p1 = 1;
    p3 = 3;
}

// normally TBase could extend from Base but Base is abstract here, hence the union
function Mixin<TBase extends (new (...params: any[]) => Derived1) | (new (...params: any[]) => Derived2)>(Base: TBase) {
    return class extends Base {
        p4 = 4;
        m() {
            console.log(this.p4);
            // error: Property 'p1' does not exist on type '(Anonymous class)'.(2339)
            // I'd expext the property to be available since both Derived1 and Derived2 have it
            console.log(this.p1);
        }
    };
}

const Derived1WithMixin = Mixin(Derived1);
new Derived1WithMixin().m();

Expected behavior: I'd expect the property p1 to be available in the Mixin since both types in the union have it.

Actual behavior: I get the compiler error error: Property 'p1' does not exist on type '(Anonymous class)'.(2339).

Playground Link: Link

Related Issues:

Start with the linked TypeScript Playground reproduction and verify the reported error for TypeScript 4.1.0-dev.20201015. Trace the compiler behavior for a mixin whose base constructor type is a union, then determine whether the shared property should be available. Done means the reproduction behaves as expected and coverage verifies the corrected behavior.