This should compile but fails in javac 8 b121:

  Stream<Optional<? extends U>> s = null;
  Stream<U> s2 = s.map(Optional::get);

error: incompatible types: inference variable R has incompatible bounds
  Stream<Number> s2 = s.map(Optional::get);
                           ^
    equality constraints: Number
    lower bounds: ? extends Number
  where R,T are type-variables:
    R extends Object declared in method <R>map(Function<? super T,? extends R>)
    T extends Object declared in interface Stream

Looks like there's a failure to capture '? extends Number', as specified by JLS 15.13.1: "If the ReferenceType [Optional] is a raw type, and there exists a parameterization of this type, T, that is a supertype of P1 [Optional<? extends U>], the type to search is the result of capture conversion (5.1.10) applied to T"