Loosening requirements for super() invocation

[Archie Cobbs]

This is an old thread which I've been looking at again lately.

To summarize, the idea is to relax the JLS rules about code prior to
super()/this() invocation to more closely match what the JVM allows, which
is field assignments and any code that doesn't reference 'this'. Relevant
issues are JDK-8194743 <https://bugs.openjdk.org/browse/JDK-8194743> and
JDK-8193760 <https://bugs.openjdk.org/browse/JDK-8193760>.

In order to make this idea less theoretical, I've prototyped all the
required compiler changes. There is also a write-up describing them
<https://github.com/archiecobbs/jdk/blob/JDK-8193760/README-JDK-8193760.md>.

There are a few interesting subtleties, e.g. relating to initialization
blocks, and there is also a sub-question, which is whether to allow
invoking super()/this() within a try block (this would have to be under the
restriction that any catch or finally clause must not return normally).
Currently that's not possible without a small JVM change, which I also
think might be worth considering to really round out this new feature. See
the writeup for details.

To see some examples of what would be allowed, see this unit test
<https://github.com/archiecobbs/jdk/blob/JDK-8193760/test/langtools/tools/javac/superInit/SuperInitGood.java>.
The compiler changes are here
<https://github.com/openjdk/jdk/compare/master...archiecobbs:jdk:JDK-8193760>
.

Thoughts?

-Archie

On Thu, Jan 17, 2019 at 8:42 AM Brian Goetz <brian.goetz at oracle.com> wrote:

> Some things have improved for this feature since we last talked; several
> verifier issues that this would have pushed on have been resolved.  So it’s
> moved from the “way too expensive for the benefit” category into the “there
> are lots of things we can do, is this really what we want to spend our
> effort and complexity budget on” category.
>
> My view on this is that while there’s nothing wrong with it, it’s also a
> pretty minor wart.  If this fell out of a bigger feature, I’d certainly not
> object, but I’d rather spend the effort and complexity budget on things
> that have broader benefit.
>
> > On Jan 16, 2019, at 5:48 PM, Archie Cobbs <archie.cobbs at gmail.com>
> wrote:
> >
> > I'm curious what are people's current thoughts on loosening the
> > requirements for super() invocation in the context of Amber, e.g.:
> >
> >    public class MyInputStream extends FilterInputStream {
> >        public MyInputStream(InputStream in) {
> >            if (in == null)
> >                throw new IllegalArgumentException("null input");
> >            super(in);      // look ma!
> >        }
> >    }
> >
>

-- 
Archie L. Cobbs
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