class BaseClass {/* ... */}

/* Type error highlighted on "Composed" */
class Composed extends BarMixin(FooMixin(BaseClass)) {}

type Constructor<I = {}> = new (...args: any[]) => I;

interface EmptyInterface {}

/** Mixin */
function FooMixin<C extends Constructor>(Base: C) {
    return class extends Base {
        private _referenceToEmptyInterface: EmptyInterface = {}; // Causes the error.  Why?
        private _fooPrivate: number = 0; // Does not cause an error.  Why not, given the above?
        protected _fooProtected: number = 0;
    }
}

/** Type of class that creates instances of type Foo */
type FooConstructor = typeof FooMixin extends (a: Constructor) => infer Cls ? Cls : never;

/** Mixin that can only extend subclasses of Foo */
function BarMixin<C extends FooConstructor>(Base: C) {
    return class extends Base {
        barMethod() {
            // We require `Base` to implement `Foo` because we need access to protected `Foo` properties.
            this._fooProtected++;
        }
    }
}