Wildcard capture with self-referencing bound

[Zhong Yu]

Elias Vasylenko posted this question on
http://stackoverflow.com/questions/30788366

Given declarations

    class C1<T extends C1<T>> {}
    class C2<U> extends C1<C2<U>> {}

Are the following parameterization well-formed?

    C1<? extends C2<String>>
    C1<? extends C2<?>>

According to his interpretation of JLS, which I concur, they are not
well-formed, because the bound of the capture variable, T#1, is C2<String>
& C1<T#1>, which is forbidden by both section $4.9 and $5.1.10.  Nothing in
JLS will infer that C2<String> <: C1<T#1>.

However, javac does allow them, which is reasonable, of course. How do we
understand this behavior?

Another question, if Ck in $4.9 contains wildcard like C2<?>, how can we
use it as the superclass of the notional class? How do we derive members of
the notional class?

Thanks,
Zhong Yu
bayou.io
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