[lworld] RFR: 8359370: [lworld] allow instance fields of identity classes to be readable in the prologue phase [v15]

[Maurizio Cimadamore]

On Mon, 1 Sep 2025 18:30:56 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>>> ok, so I guess I still don't get whether this _must_ be an error. In principle `y` could have a local proxy, in which case the lambda could be thought of as accessing that proxy, so no need to capture `this` ?
>> 
>> I wonder what is the mental model supposed to be here.
>> 
>> @mcimadamore what is your opinion on whether this should compile?
>> 
>> class A {
>>     int y;
>>     A() {
>>         y = 1;
>>         class B {
>>             static void m() {  // static context
>>                 System.out.println(y);
>>             }
>>         }
>>         super();
>>     }
>> }
>> 
>> If your answer is "No" then aren't you then implying that `y` shouldn't be available whenever `A.this` is not available? In which case doesn't that answer your question?
>> 
>> If your answer is "Yes", then doesn't that imply that _this_ should also compile...
>> 
>> class A {
>>     int y;
>>     A() {
>>         y = 1;
>>         class B {
>>             static void m() {  // static context
>>                 System.out.println(A.this.y);
>>             }
>>         }
>>         super();
>>     }
>> }
>> 
>> even though this doesn't:
>> 
>> class A {
>>     int y;
>>     A() {
>>         y = 1;
>>         class B {
>>             static void m() {  // static context
>>                 System.out.println(A.this);
>>             }
>>         }
>>         super();
>>     }
>> }
>
> I suppose what I'm saying is: I understand why the code doesn't compile in today's world. But as we relax more restrictions and we resort to more complex translation strategies, I do wonder if some of these rules that prevent reads from lambdas will feel too tight. E.g. imagine the case of a final field -- that is written only once. If we already saw a write for that field, what stops us from being able to reference it from a lambda -- through a local proxy?
> 
> I don't buy the argument that `A.this.y` working implies `A.this`. This is already not the case in the code added by this PR, where reading an already written field in a prologue is fine, even through `A.this.y` -- but accessing `this` of a class from the prologue is never ok (if it was you could pass such a larval `this` to another method).

(in your example with static context, my answer is that no, it should _not_ compile. A static context can't access instance fields from an enclosing class)

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1523#discussion_r2314448628