Incoherent invocation type inference?

[B. Blaser]

Hi,

2017-01-13 15:12 GMT+01:00 Maurizio Cimadamore <maurizio.cimadamore at oracle.com>:
> I think that's correct behavior. When you say something like:
>
> li = get(null);
i = get(null); // li = ... is OK ;-)

> T will get two constraints:
>
> * an upper bound (from declared bound) : T <: Iterable<String>
> * an upper bound (from return type compatibility) : T <: Integer
Integer being final, a correct return type constraint would be <T =
Integer>, leading to incompatible bounds {T <: Iterable<String>, <T =
Integer>} and then javac should fail...?

Bernard

> The language will then infer T = Integer & Iterable<String>, an intersection
> type.
>
> Normally this would be fine - but the issue here is that Integer is a final
> class, so that intersection type has no witnesses - e.g. there's no way for
> get() to construct a type that is both an Integer and an Iterable<String> -
> so that will almost always fail - unless get() returns null.
>
> That said, I'm not aware of any check in the spec for preventing an
> intersection type to mention a final class as one of its components so,
> while this does look weird, I think the compiler is doing what the language
> spec says.
>
> Maurizio
>
>
>
> On 13/01/17 13:01, B. Blaser wrote:
>>
>> mport java.util.*;
>>
>> public class Issue  {
>>      <T extends Iterable<String>> T get() { return (T)new
>> ArrayList<String>(); }
>>      <T extends Iterable<String>> T get(T t) { return (T)new
>> ArrayList<String>(); }
>>
>>      void run() {
>>          List<String> li = null;
>>          LinkedList<String> ll = null;
>>          Integer i = null;
>>
>>          li = get(null);
>>          i = get(null); // Shouldn't compile? and will fail at runtime.
>> //        i = get(li); // Fails as expected
>>
>>          li = get();
>>          ll = get(); // OK, but will fail at runtime due to unchecked
>> conversion in get().
>>          i = get(); // Shouldn't compile? and will fail at runtime.
>>      }
>>
>>      public static void main(String... args) {
>>          new Issue().run();
>>      }
>> }
>
>