package p1;
class C1 {
  void m ();
}

package p2;
class C2 extends C1 { // error message here
}

Currently IDEA displays error message: Class C2 must be either declared abstract or implement method 'm' in 'p1.C1'
This message is displayed even if method m is present in C2.

Javac is just as unhelpful in this situation, but I think IDEA could do better. Ideally, it should:

• insist that C2 absolutely must be abstract (no option to implement method)
• if the method is present in C2, higlight it as error or warning (needs more thought)