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Malte UblandClaude Opus 5.5
ed50daa853
An optional parameter typed by a union that is still being mapped gets a
recursive placeholder: a union id whose arms arrive when the outer frame
finishes. The function-type mapping then armed that placeholder with
undefined, but the placeholder's arm list was still empty, so the result
was a fresh union holding only `undefined`. The parameter lost its record
arm, the same checker type mapped to two different layouts depending on
where the mapping started, and later conversions failed (SC2001, SC2002,
SC2003).
This happens when an interface method takes an options object whose
callbacks are typed by a subinterface of that same interface:
`newSession(options?: { withSession?: (ctx: Replaced) => Promise<void> })`
with `interface Replaced extends CommandContext`.
When the checker spelled the parameter with an undefined constituent, the
placeholder already carries that arm, so the parameter keeps the
placeholder. withUnitArm now refuses a pending placeholder instead of
building the unit-only union.
pi's ExtensionCommandContext is such an interface. It refused
ExtensionAPI.registerCommand, and with it pi's main().
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>