Reports type variables used as arguments in locations that their variance does not allow, so long as they are not marked as unchecked. A type variable may have a variance of either covariant (out), contravariant (in), or invariant (neither out nor in).

The following rules are used to check the type arguments used in attribute and method declarations:

unchecked type arguments are ignored.
Covariant type arguments may only be used in locations that produce values (for example, return types and block parameters).
Contravariant type arguments may only be used in locations that consume arguments (for example, parameters and block return types).
Invariant type arguments may be used in both locations that produce values and consume arguments.

Example:

class Example[in A, out B]
  # Covariant type variable 'B' of class 'Example' used in a contravariant position, ...
  def call: (B) -> A
end