+1
That fix the bug

My change caused another issue so you might want to close this.

see #379 (comment)

Instead of closing the Pr, you can convert the PR to WIP (Work In Progress) by modifying the title and add WIP as a prefix, and take your time to resolve the side effect.

I think these methods are needed to be added:

  object  GetDefaultValue(this OptionSpecification option )
  {
    //get the default value from the specification of option
  }

bool  HasDefaultValue(this OptionSpecification option )
  {
    //if  the option has a default value from specification return  true otherwise return false
  }

and IsEmpty() can be based on the computation of these methods.

I see IsEmpty() is calculated from the default value of the BaseType and it should also calculated based on its spec Default.

L252

        if (value is ValueType && value.Equals(value.GetType().GetDefaultValue())) return true;

Sure, I'll give it a try this week or the next

The best I could do was to change it to:
if (!spec.Required && value is ValueType && value.Equals(value.GetType().GetDefaultValue())) return true;
This solves the issue by always including required value types, and skipping non-required ones which have a default value.

At first I tried replacing the line with this:

if (spec.HasDefaultValue() && value.Equals(spec.DefaultValue.FromJust())) return true;

but it broke some UnParserExtensionsTests.

For example:

UnParsing_instance_returns_command_line

Message: Expected string to be equivalent to 
"-i 1 2 3" with a length of 8, but 
"-i 1 2 3 0" has a length of 10.

The BaseType default was included even though it wasn't supposed to be.