`Array.isArray(v)` is a compile-time constant when `v` has a static type:
an `T[]` or a tuple is an array, every other kind is not. The fold only
accepted a bare variable, a record field or a field read, because only
those could be dropped without losing effects, so `Array.isArray(make())`
and `Array.isArray([1, 2])` reached an SC1090 refusal.
A computed argument now folds the same way after being evaluated for its
effects: the value goes into a sequence whose discarded statement is the
argument. Unions keep answering by their runtime tag, and `unknown`
values keep their real runtime test.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
