OT/J Language Definition v1.3.1

§2.3.4.(a) Potential ambiguity

A potential ambiguity is given, if two role classes R1 and R2 exist such that

R1 and R2 are played by the same base class B, and
R1 and R2 have a common super role R0, which is also bound to a base class B0, and
neither role class R1 nor R2 is a (indirect) sub-class of the other.

Note:

According to §2.1.(c), if B is distinct from B0 it has to be a sub-class of B0.

Effect:

In this case the compiler issues a warning, stating that the B may not be liftable, because both role classes R1 and R2 are candidates and there is no reason to prefer one over the other.
If no potential ambiguity is detected, lifting will always be unambiguous.

In the above situation, trying to lift an instance of type B to the role type R0 is an illegal lifting request. If R0 is bound to the same base class B as its sub-roles R1 and R2 are, role R0 is unliftable, meaning that no instance of R0 can ever by obtained by lifting.

Example code (Potential Ambiguity):

team class MyTeam {
  public class SuperRole playedBy MyBase {...}
  public class SubRoleA extends SuperRole {...}
  public class SubRoleB extends SuperRole {...}
}

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§2.3.4.(b) Definite ambiguity >>

1	team class MyTeam {
2	public class SuperRole playedBy MyBase {...}
3	public class SubRoleA extends SuperRole {...}
4	public class SubRoleB extends SuperRole {...}
5	}