Consider the following code (try):

class C {
  constructor() {
    this.method = this.method.bind(this);
  }
  //method: () => void; // uncomment this line
  method() {}
}

Which generates the following error:

3:     this.method = this.method.bind(this);
            ^ property `method`. Covariant property `method` incompatible with contravariant use in
3:     this.method = this.method.bind(this);
       ^ assignment of property `method`

As noted in the code, if you add a separate property type declaration, the error is gone. IMO, it does not really make sense to enforce a separate declaration here as the method definition should be enough?