The error message for incompatible inherited methods doesn't account for the possibility that both methods could come from the same class. The message could be improved:

InstanceClash.java:13: error: types Foo<String> and Foo<String> are incompatible; both define m(java.lang.String), but with unrelated return types
  interface I extends Foo<String> {}
  ^
Here's the relevant source:

  public static interface Foo<S> {
    String m(String s);
    int m(S o);
  }

  interface I extends Foo<String> {}