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Java generics mystery

Here is a puzzle for Generics gurus.

Can somebody explain why

Set<Address> addresses = new HashSet<Address>();
Set<?> things = addresses;

compiles but

Set<ConstraintValidator<Address>> validatedAddresses = 
    new HashSet<ConstraintValidator<Address>>();
Set<ConstraintValidator<?>> validatedThings =
    validatedAddresses;

does not compile?

More specifically, the assignment on the second line breaks.