[m-rev.] for post-commit review: add comments to typecheck_coerce.m

[Zoltan Somogyi]

On Sat, 25 Jul 2026 15:37:34 +0200 (CEST), "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> For review by Peter, mostly just of the new NOTE about
> existentially typed args in data constructors.

That note is:

    % NOTE If the base type_ctor has arity zero (meaning it has no parameters),
    % we could shortcut this and just return an empty set as InvariantTVars,
    % IF we knew that none of its data constructors had existentially
    % typed arguments. This is because the InvariantTVars we return
    % are guaranteed to be a subset of the type_ctor's type parameters,
    % EXCEPT in the presence of such arguments.

I have since realized that the presence or absence of existentially quantified
type vars in InvariantTVars does not matter, because the only thing we use
InvariantTVars for is to test whether a given type parameter is in that set.
So the small optimization describe above should work. I will add it after my
latest diff is reviewed/committed.

Zoltan.