// Loose function or method. Same result.
declare function bar(thing: string): void;

interface Base {
    thing?: string;
}
class Example<Props extends Base> {
    props: Readonly<Props>;
    mutableProps: Props;
    base: Readonly<Base>;

    foo() {
        if (this.props.thing) {
            this.props.thing.toString(); // No error
            bar(this.props.thing); // Error!
        }
        if (this.mutableProps.thing) {
            bar(this.mutableProps.thing); // No error
        }
        if (this.base.thing) {
            bar(this.base.thing); // No error
        }
    }
}