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Chris Tate
534c08dd2f
A call that spread a fixed tuple and also an array, a Set or another
iterable into a `(T | undefined)[]` rest parameter took the general rest
path, which converts every value to T, so an undefined tuple field threw
("undefined is not representable in the target union"). Such calls now
pack every item in argument order through lowerArraySpreadItems, which
takes values already lowered (the tuple's fields) beside argument nodes
and keeps undefined values as present undefined elements.
A tuple element typed only `undefined`, as in `...["b", undefined]`, has
a `null | undefined` field. Stored into an array that keeps undefined
elements it is now evaluated and stored as undefined, where it was an
internal compiler error; the checker admits it there only as undefined.