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