TypeScript/tests/cases/compiler/classSideInheritance2.ts

interface IText {
    foo: number;
}

interface TextSpan {}

class SubText extends TextBase {

        constructor(text: IText, span: TextSpan) {
            super();
        }
}

class TextBase implements IText {
        public foo: number;
        public subText(span: TextSpan): IText {

            return new SubText(this, span);
        }
}