in Sources/_CollectionsTestSupport/MinimalTypes/MinimalDecoder.swift [438:451]
func decodeNil() throws -> Bool {
expectTrue(isValid, "Container isn't valid", trapping: true)
commitPendingContainer()
guard index < input.count else {
let context = DecodingError.Context(
codingPath: codingPath + [_Key(index)],
debugDescription: "Unkeyed container is at end")
throw DecodingError.valueNotFound(Any?.self, context)
}
let value = input[index]
guard value == .null else { return false }
index += 1
return true
}