See 10.3.1. Assignability of type variables, raw and parameterized types:

A parameterized event type is considered assignable to a parameterized observed event type if they have identical raw type and for each parameter:

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• the observed event type parameter is a wildcard and the event type parameter is assignable to the upper bound, if any, of the wildcard and assignable from the lower bound, if any, of the wildcard, or
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