`Array.from({ length: n }, () => undefined)` and `.map(() => undefined)`
type as `undefined[]`, which the type mapping represents as an array of
unit-union elements. Callback-driven producers take the element from the
lowered callback's return type, and a standalone `undefined` in return
position maps to the void convention — its answer is erased before it
could be stored — so every discarding callback was refused (SC2009; pi's
tui terminal-color palette).
A producer whose result element is the unit-only union now gives the
callback that union as its contextual return, exactly as the array
element mapping substitutes it for a `void` element: the arrow's
`undefined` answer becomes the present unit value. `[undefined, ...]`,
sparse literals and an explicit `undefined[]` annotation already had
that representation.
Corpus 8527 pins the palette field, the mapper's call order and index,
the `.map` and `Array.from(array, ...)` forms, `in`/JSON and the empty
case under Node and natively.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
