The runtime-optional analysis widens a parameter or binding that can
observe undefined with an undefined arm. For a Date it interned the
union `Date | undefined`, which is not a valid union (Date is not a
union arm kind), and the whole program failed IR validation with SC9001
"union uN: arm 0 is date" (morgan's date token and clfdate).
Like generators, a Date keeps its type: the canonical undefined-arm
helper already refuses Date, and a site that would store undefined into
the slot keeps its own diagnostic.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
