Consider the following:

type X = {
  a: number;
};
type Y = X & { b: number };

abstract class XX {
  abstract yy(x: X): void;
}

class YY implements XX {
  yy(x: Y): void {
    console.log(x.b);
  }
}

const xxs: XX[] = [new YY()];
xxs[0].yy({ a: 1 });

The code above compiles without errors, but why does TS allow a concrete class (YY) to implement an abstract method (yy) receiving a parameter of a narrower type (Y) of the type declared in the abstract class (X)?

This makes the method yy accept a X input without compiler errors though the signature and contents of the method clearly expects an Y.