test to choose overloaded method to detect target type
type equality constraint: accept unbounded/extends wildcards pair; reject unbounded/super
type compatibility constrain: boxing is not completeness-preserving: move treatment to return constraint
type compatibility constrain: unchecked conversion step
type compatibility constrain: unchecked conversion: return type is inference variable case
separate subtyping constraints
type equality constraint: reject type/wildcard pairs; subtyping constraint: become eq constraint for types
fix thread access violations