Loosening requirements for super() invocation

[Archie Cobbs]

On Tue, Jan 24, 2023 at 2:21 PM Brian Goetz <brian.goetz at oracle.com> wrote:

>
> This made me realize there is another missing requirement:
>
>    - Upon normal return from a constructor, the `this` reference must be
>    DA
>
>
> So, I think this is true by definition; on normal return from a
> super/this/implicit super call, then `this` *is* DA.
>
> You can simplify the rule about try blocks to:
>
>  - if `this` is DU on entry to a try block, it must be DU on normal
> completion of the try block.
>
> Can you simplify further, while still preventing `this` calls inside a try
> block?  I don't think so, since we might want to allow a try block before
> the super/this call.
>

Ok thanks. I admit I'm getting a little out of my league here.

Does this sound correct?

 The JLS will be modified as follows:

 * Remove the requirement that `super()` or `this()` appear as the first
statement in a constructor
 * Add superclass initialization to the existing constructor dataflow
analysis as follows:
     * The `this` reference is considered DU on entry to the constructor
     * Before a `this()` or `super()` call, the `this` reference must be DU
     * After a `this()` or `super()` call, the `this` reference is
considered DA
     * Before normal completion of the constructor, the `this` reference
must be DA
     * If `this` is DU on entry to a `try` block, it must be DU on normal
completion of the `try` block
     * If no explicit `this()` or `super()` appears in a constructor, it is
treated as if the first line of the constructor were `super()`
 * Add the following restrictions on constructors:
     * No access to `this`, other than assignments to fields, may occur
unless `this` is DA
     * `super()` and `this()` may not appear within any `try { }` block
 * Clarify that non-static field initializers and initialization blocks are
executed immediately after `super()` invocation, wherever it occurs (see
"Initialization Order" below)


Thanks,
 -Archie

-- 
Archie L. Cobbs
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