Loosening requirements for super() invocation

[Brian Goetz]

The basic design seems sound enough.  Probably time to dig to the next 
level, which is assessing the spec impact.

The places to start are Ch16 (definite assignment) and 8.8.7 
(constructor bodies).

On 1/24/2023 4:24 PM, Archie Cobbs wrote:
> 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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