Double-dash options that take a value should use either white-space or an '=' to separate the name from the value, as in
    --name value
    --name=value

However, there are 3 hidden options that deliberately go out of their way to subvert that policy, and impose a hybrid of the old and new syntax rules. These options use a compulsory ':' to separate the name from the value. This needs to be fixed.

The options are: --debug: --should-stop: and --diags:. When these are fixed to be syntactically standard double-dash options, the internal class HiddenGroup can/should be removed.