Is V? what we need ?

Maurizio Cimadamore maurizio.cimadamore at
Thu May 2 11:16:39 UTC 2019

On 01/05/2019 20:34, Brian Goetz wrote:
> ne big difference between whether we convert V to V? via subsumption 
> or via a boxing conversion is how overload selection is done.  But, if 
> we have methods m(Object) and m(V?), and it is invoked with a v, in 
> the subtyping case, we'll select m(Object), and in the conversion 
> case, we'll select m(V?).  But it's not clear that this is what we want

There's a tiny gap in this line of reasoning.

All these arguments rely on the fact that V <: Object, I realized. 
While, again, this might be true from a VM perspective, it doesn't have 
to be true for the language. We can be in a world where:

V? <: Object


V ~> V?

That is, the first relation is covered by subtyping, while the latter 
involves a conversion (albeit a trivial one).

If you go down this path, overload resolution works the way you expect: 
if you wanna pass a V to a m(Object) or m(V?) you require a conversion 
anyway. So m(V?) wins because it's most specific.

If you wanna pass a V to m(V) or m(V?), m(V) wins, and m(V?) wouldn't 
even be considered.

All this boils down, I think, to one question: do we want array 
covariance? E.g. should

V[] <: Object[]


If yes, then we need to have V <: Object (which then implies also V <: 
V?). If not, then we can use conversions as described above.

Another factor that might inform the decision is wildcard treatment (as 
covariance is unsurprisingly related to arrays) - do we want:

List<? extends V> <: List<? extends Object> ?

These are, IMHO, they key questions - once we answer them unambiguously, 
all else fall as a sequence of basically forced moves, I think.


If we're really appealing to int vs. long, as John noted, int[] and 
long[] are unrelated types currently, so as int[] and Object[], which 
seems to push against covariance. I also remember some arguments 
exchanged about subtyping between primitive types being a 'bug' in the 
current JLS - e.g. int <: long is more a bug than a feature, but I could 
be remembering incorrectly.


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