OverlayPick

type OverlayPick =
  | {
  meta?: never;
  option: CoreOption;
  value?: never;
}
  | {
  meta?: string;
  option?: never;
  value: string;
};

Defined in: core/src/shared/types.ts:142

WHAT AN OVERLAY ACCEPTS, and a UNION because the two arms are exclusive in fact: naming a row KIND retypes the caret’s row, naming a VALUE writes the trigger option’s markup, and no call does both. Spelled as one optional bag the illegal states were representable — {} wrote @[]() into the document, and {option, value} typechecked while silently dropping value.

The ?: never members are load-bearing: a bare union does NOT forbid {option, value}, because excess-property checking against a union accepts any key declared by ANY arm.