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List:       openjdk-compiler-dev
Subject:    Re: spec clarification: wildcard array element signature
From:       Dan Smith <daniel.smith () oracle ! com>
Date:       2017-02-07 20:57:54
Message-ID: 69AC195E-A1C8-46FB-B771-2C19E8A2EF61 () oracle ! com
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> On Feb 7, 2017, at 12:38 PM, Alex Buckley <alex.buckley@oracle.com> wrote:
> 
> On 2/7/2017 10:54 AM, Dan Smith wrote:
> > Yes, I object! "?[]" is not a type. It's meaningless.
> > 
> > It is always wrong to perform substitutions that replace type
> > variables with wildcards. javac has a history of doing so, and those
> > cases need to be fixed. The compiler's internal representation of
> > wildcards should not even allow such an attempt at substitution to
> > compile (e.g., a class Wildcard should not extend a class Type), but
> > alas, we have a lot of legacy violating this rule.
> 
> Am I right to say that this is what javac is doing in the recent snipper from \
> Stephan Herrmann? :- 
> ---
> public class Bug494198<T> { class Inner<U> {...} }
> 
> Bug494198<?> outer = ...
> ... outer.new Inner<String>() {...}
> 
> final class Bug494198$1
> extends ***Bug494198<<captured wildcard>>***.Inner<java.lang.String>
> ---

Yep, same issue. Odd that javac uses capture in this case while using a wildcard in \
the other case, but it comes down to the same lack of a spec.

> > Liam's program raises issues related to two existing spec bugs:
> > JDK-8030746: 4.10: Define subtyping for inner classes of
> > parameterized types JDK-8016196: Inference: define supertype
> > parameterization for wildcard-parameterized types
> > 
> > These are part of our type system cleanup effort; when addressed,
> > we'll have a clear answer for how to interpret "B<?>.I" and, thus,
> > how to record it in bytecode. (Preview: the answer is probably "A<?
> > extends Object[]>.I".)
> > 
> > Suggest closing JDK-8172742 as "Not an Issue"—the JVMS grammar is
> > fine.
> 
> OK -- please close it with an explanation for Liam and Stephan's benefit.

Done.

—Dan


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