I am currently writing a Java compiler and have implemented section 15.12.2.7. of the JLS7 (http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.12.2.7) , one of the most annoying sections of the spec. I still have one problem since the spec somehow seems underspecified or ambiguous. My problem is this line:
lcta(U) = ? if U's upper bound is Object, otherwise ? extends lub(U,Object)
U is an arbitrary type expression. What is the upper bound of a type expression? In addition, why is the lcta always a wildcard?
The spec defines
CandidateInvocation(G) = lci(Inv(G)).
Now, for example, consider the case that Inv(G) = { List<String> }, i.e., the only possible candidate invocation is a single parameterized type. Now, due to the rule
lci(G<X1, ..., Xn>) = G<lcta(X1), ..., lcta(Xn)>,
the result of CandidateInvocation( G ) = lci( { List<String> } ) would be defined as:
List<lcta(String)>
in my opinion, lcta should simply return String here, because if List<String> is the only possible invocation, it is a good idea to infer List<String> as the argument. However, the definition of lcta(U) dictates that the result is either ? or ? extends lub(...), so the result IS ALWAYS a wildcard. This seems strange. What am I misinterpreting here?
