[Boost-users] Bug when using boost::optional with boost::program_options?

Subject: [Boost-users] Bug when using boost::optional with boost::program_options?
From: Jim Porter (jvp4846_at_[hidden])
Date: 2018-06-02 04:53:06

Before I spend too long tracking this down (I'm only seeing it in an 
Appveyor Windows build, since I do most of my work on Linux), I wanted 
to see if anyone else has experienced this issue. I'm trying to use 
boost::program_options to parse a command line arg and store it into a 
boost::optional, however I get a surprising compilation error[1].

This appears to have been introduced sometime between Boost 1.60 and 
1.62 (Appveyor doesn't have Boost 1.61). I see that there were some 
significant changes to boost::optional between those revisions, so maybe 
that's the cause? The following test case *might* reproduce the issue, 
but sadly, rextester only has Boost 1.60, so it works there[2]. If it 
fails for someone with Boost 1.62+, then great! (Well, not "great", but 
you know what I mean.)

Interestingly, it's not until Boost 1.65[3] that boost::program_options 
claims to support storing values into boost::optional; it's worked for 
me since long before that, possibly because I `#include 
<boost/optional/optional_io.hpp>`.

If this isn't enough for others to help confirm this issue, that's ok; 
I'll try to get a Windows dev environment set up to provide more details 
in the coming week. I just wanted to send this out before I forgot about 
it...

If it matters, the Appveyor build I'm seeing this with uses Visual 
Studio 2015 Update 3.

Thanks,
- Jim

[1] https://hastebin.com/awutivawew.txt
[2] http://rextester.com/QGBH48883
[3] https://www.boost.org/users/history/version_1_65_0.html