[boost] [program_options] implicit value and non-adjacent tokens

Subject: [boost] [program_options] implicit value and non-adjacent tokens
From: Vladimir Prus (vladimir.prus_at_[hidden])
Date: 2016-12-21 02:36:46

Hi,

some time ago, program_options was modified so that options
with implicit value would only take value from the same token.

Previously, one could have:

        ./program -a foo -b # uses 'foo' as value for 'a'
        ./program -a -b     # uses some default value for 'a'
        ./program -b        # has no value for 'a'

Now, the first syntax interprets 'foo' as positional option,
and one has two write

        ./program -a=foo

Ever since, I hear concerns from users about this behavior. Maybe not 
very loud, but quite consistent. Maybe, the new behavior is not
very good, indeed, because it now mixes syntax matter (whether value
is in the same token) with semantic matter (whether an option has
implied value).

I am considering reverting this change for next version of Boost.
Does anybody have opinions either way?

Thanks,
Volodya