Expressions - The Java Language Specification - page 476

We have A = G < W > >> F = G < U > for some type expression W . Since W is a
type expression (and not a wildcard type argument), it must be the case that W
= U , by the invariance of parameterized types.
Otherwise, if A is of the form G < ..., X k-1 , ? extends W , X k+1 , ... > , this algorithm
is applied recursively to the constraint W >> U .
We have A = G <? extends W > >> F = G < U > for some type expression W . It must
be the case that W >> U , by the subtyping rules for wildcard types.
Otherwise, if A is of the form G < ..., X k-1 , ? super W , X k+1 , ... > , this algorithm is
applied recursively to the constraint W << U .
We have A = G <? super W > >> F = G < U > for some type expression W . It must
be the case that W << U , by the subtyping rules for wildcard types.
Otherwise, no constraint is implied on T j .
• If F has the form G < ..., Y k-1 , ? extends U , Y k+1 , ... > , where U is a type expression that
involves T j , then:
♦ If A is an instance of a non-generic type, then no constraint is implied on T j .
Once again restricting the analysis to the unary case, we have the constraint A
>> F = G <? extends U > . A must be a supertype of the generic type G . However,
since A is not a parameterized type, it cannot depend upon U in any way. It is
a supertype of the type G <? extends X > for every X such that ? extends X is a valid
type argument to G . No meaningful constraint on U can be derived from A .
♦ If A is an invocation of a generic type declaration H , where H is either G or su-
perclass or superinterface of G , then:
If H ≠ G , then let S 1 , ..., S n be the type parameters of G , and let H < U 1 , ..., U l >
be the unique invocation of H that is a supertype of G < S 1 , ..., S n > , and let V =
H <? extends U 1 , ..., ? extends U l >[ S k = U ] . Then this algorithm is applied recurs-
ively to the constraint A >> V .
Our goal here is once more to simplify the relationship between A and F , and
recursively invoke the algorithm on a simpler case, where the type argument
is known to be an invocation of the same generic type as the formal.
Assume both H and G have only a single type argument. Since we have the
constraint A = H < X > >> F = G <? extends U > , where H is distinct from G , it must
be the case that H is some proper superclass or superinterface of G . There must
be an invocation of H < Y > , such that H < X > >> H < Y > , that we can use instead of
F = G <? extends U > .