Return type inference: no unique maximal instance exists difference between 1.6 and 1.7 javac.

[Dawid Weiss]

Thank you very much, Maurizio -- this is exactly what I was looking for.

Dawid

On Mon, Nov 29, 2010 at 12:31 PM, Maurizio Cimadamore
<maurizio.cimadamore at oracle.com> wrote:
> Hi Dawid
>>
>> 1) Is the difference between javac 1.6 and 1.7 a bug in 1.6's
>> compiler? I checked a few past snapshots for 1.7 and they all worked,
>> so it must be a change done a good while ago. If this was a bug at
>> some point, is there a bugs parade entry for it anywhere (I did search
>> for something matching this, but couldn't find anything).
>>
>
> This is a bug in javac 6 - see:
>
> http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6302954
>
> [as you noted this one got fixed very early in 7]
>>
>> 2) We both tried to formally prove the above code correct or incorrect
>> wrt the Java specification, but the complexity of type inference
>> rules, especially in assignment expressions, is, ehm, overwhelming. We
>> would appreciate if somebody could explain what the actual process of
>> inferring the type of B (or A) is in the above example;
>>
>
> Type inference is a process structured in two steps:
>
> 1) first, generic method type-arguments are inferred from the actual
> argument types supplied in the method call - this section is covered in JLS
> 3rd 15.12.2.7; since there are no actuals here, it doesn't apply in this
> particular case.
>
> 2) Secondly, any uninferred type-variable is inferred by collecting
> constraints from (i) declared bounds of type-variables and (ii) expected
> type in assignment context. This rules are described in JLS 3rd, 15.12.2.8.
>
> If we apply (2) in this particular case:
>
> public<A>  A x() { ... }
>
> public<B>  void y() {
>   B o = x(); //we need to infer a type for A
> }
>
>
> We have that A's bound is Object. Also, from the assignment context, we have
> that the inferred type for A must be a subtype of B (written A <: B).
>
> More formally:
>
> A <: Object
> A <: B
>
> Section 15.12.2.8 says that the inferred type for A is the glb() of all the
> constraint we were able to collect:
>
> A = glb(Object, B) = B
>
> Which means that, in this example, the method call to x() is legal and the A
> type-variable should be inferred to be B.
>
> Maurizio
>
>
>
>