Possible records tweak

[Dan Smith]

> On Apr 29, 2020, at 11:28 AM, Guy Steele <guy.steele at oracle.com> wrote:
> 
> Appealing, but it does raise the question of whether, if the programmer uses some sneaky trick such as calling super, this can really be enforced ar compile time on all cases. If it can (after all), then your proposed wording looks good to me. 

The idea would be to rely on the definition of "assign to" in JLS 16. That uses a heuristic that counts "x =" or "this.x =" (and, in javac, but not currently specified properly, "(this).x", etc.)

Stepping back: in general, it's illegal to assign to a final field. There's one exception: inside a constructor, where the field is DU, using an assignment that satisfies the heuristic. In any other case, you get an error.

So the language has carved out a small hole permitting assignments, and this rule closes that hole.