Java 8 compiler bug?

[Oliver Gierke]

Hi Maurizio,

answer inline…

> Am 17.11.2016 um 13:44 schrieb Maurizio Cimadamore <maurizio.cimadamore at oracle.com>:
> 
> On 15/11/16 23:46, Oliver Gierke wrote:
>> CriteriaBuilder and all other types come from JPA. The method in question has two overloads:
>> 
>> <E, C extends Collection<E>> Predicate isNotMember(Expression<E> elem, Expression<C> collection);
>> <E, C extends Collection<E>> Predicate isNotMember(E elem, Expression<C> collection);
>> 
>> I fail to understand why the second method is even considered as that create a contradiction between the bound values of E (Expression<…>) and C (Collection<Object>).
> Hi Oliver,
> I believe the JDK 8 behavior is correct here - let's just consider your second method - which is the one you have doubts about:
> 
> <E, C extends Collection<E>> Predicate isNotMember(E elem, Expression<C> collection);
> 
> 
> Now, you want to pass the following actual argument types to this method:
> 
> arg1 = Expression<Object>
> arg2 = Expression<Collection<Object>>
> 
> So, let's look at which constraints these argument cause trigger on our inference variables E and C.
> 
> So, since Expression<Object> must be compatible with E - we have that
> 
> Expression<Object> <: E
> 
> Then, since also Expression<Collection<Object>> must be compatible with Expression<C> we have that:
> 
> Expression<Collection<Object>> <: C

Is that correct? If the method parameter takes an Expression<C> and we hand in an Expression<Collection<Object>>, C should be bound to Collection<Object>, not Expression<Collection<Object>>, right?

This basically where my confusion comes from, as that would then render E to evaluate to Object for the second parameter which then contradicts the E derived for the first one, right?

Cheers,
Ollie

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