> On Nov 22, 2017, at 6:48 AM, Brian Goetz <brian.go...@oracle.com> wrote: > > What's the L-world story for array subtyping? For any R-type, R[] <: > Object[]. If everything is an L type and everything is <: Object, are arrays > of Q-types/primitives also subtypes of Object[]? > > We didn't have a story for this in QU-world either, but at least in QU-world > it was believable that QFoo[] <! Object[]. But that seems much less tenable > when there's no syntactic difference between L-uses and Q-uses. (And even > less so when we might migrate code from L to Q.)
My two cents: we didn't discuss this in depth, but John raised it in this thread, and in the "design notes" document, I followed up with some details. - Initially, QFoo[] is not a subtype of LFoo[]. You want covariant subtyping, you need to stick with L types. - As an enhancement, we can introduce covariant Q-L subtyping, adjust the behavior of aaload/aastore, and explore the performance impact. What's hard about treating QFoo[] as an LObject[] is that the layout is a dynamic property, requiring dynamic checks. But we may also be interested in pursuing non-uniform layout for QFoo[] (a specific idea: flattening generally but not for "volatile" instances), so there may be some satisfactory coping techniques coming. On Java syntax: who says there's no syntactic difference between L-uses and Q-uses? You might spell the L-use "Complex?". But, anyway, that's a question to raise in a year, after we better understand the JVM. —Dan
