But then the synopsis is tool --opt=VALUE [FILE] which is something else and not what the cookbook is talking about.
In what I’m talking about the synopsis is --opt[=VALUE] [FILE] and tool --opt is meaningful so the user “is not using it wrong”.
(I think you are confusing the notion of default value taken in your program if the option is not specified on the cli and the ability to have on the cli an option name that takes a value but that can be left unspecified).