Java 8 compiler bug?

[Remi Forax]

I think the main issue is that when the inference fails, the compiler instead of reporting an error continue using Object instead.

Remi


On November 17, 2016 5:12:19 PM GMT+01:00, Maurizio Cimadamore <maurizio.cimadamore at oracle.com> wrote:
>
>
>On 17/11/16 13:56, Oliver Gierke wrote:
>> 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
>>
>Let me reboot, as I made a bit of a mess :-)
>
>You have:
>
>Expression<Object> <: E
>
>
>and
>
>Expression<Collection<Object>> <: Expression<C>
>
>
>
>The latter constraint produces:
>
>C = Collection<Object>
>
>And then you also have:
>
>C <: Collection<E>
>
>Then from transitivity you get
>
>Collection<Object> <: Collection<E>
>
>--> E = Object.
>
>Note that this answer is correct - as we needed a type for E that was 
>_bigger_ than Expression<Object>, and Object is obviously one such type
>
>(being Object the biggest possible type).
>
>So, while I made a mistake in my derivation, the conclusion is still 
>valid: the treatment of transitivity (new in Java 8) allows this 
>inference to succeed, while the lack of it caused inference to fail in 
>Java 5, 6,7.
>
>Maurizio

-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.
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