type APIApplicationCommandSubcommandGroupOption = {
    options?: APIApplicationCommandSubcommandOption[];
}

type CommandSubcommandGroupOption = Omit<APIApplicationCommandSubcommandGroupOption, 'options'> & { // <-- so is this one
  options?: CommandSubcommandOption[]
}

type CommandSubcommandOption = { otherProps: unknown } & { // <-- this intersection is apparently critical
  options?: unknown[]
}

interface APIApplicationCommandSubcommandOption {
    options?: { foo: string }[];
    otherProps: unknown;
}

function transformSubcommandGroupOption1(option: CommandSubcommandGroupOption): APIApplicationCommandSubcommandGroupOption {
  return option; // no error, but there _should_ be an error here.
}

function transformSubcommandGroupOption2(option: CommandSubcommandGroupOption): APIApplicationCommandSubcommandGroupOption {
  return { ...option }; // error in 3.9.7
}