optional support (and boost configuration) issue

[jessey-git]

This issue has 2 parts:

• It appears as though the single-file, combined, CLI11.hpp file is built by first assembling the CLI/Optional.hpp file above the CLI/Version.hpp file. This has the side effect that some defines like CLI11_CPP17 won't be set yet when Optional.hpp starts looking over them.

• There's no way to disable boost support. The library goes out of its way to detect boost but in my case I kind of don't want it to do that :) How do I ensure CLI11 doesn't drag in boost headers when I don't want it to? Maybe use an ALLOW_BOOST define that the user can set before including CLI11.hpp so the lib knows not to check at all.

[henryiii]

Fixed in master, and v1.5 branch (I consider this important enough to release a patch release). Please test!

I've fixed the include order issue - it was broken by adding the verbatim sections; the best fix was to put the Version and Macros code in verbatim sections too.

You can now define CLI11_BOOST_OPTIONAL to 0 to avoid the search. 1 forces the inclusion, and undefined will do the search.

Note: one side effect: you have to do

#define CLI11_BOOST_OPTIONAL 1

to force this in user code; before you could forget the 1.

[jessey-git]

Both issues seem fixed for me. Thanks!

However, I've now noticed a third issue for MSVC. The usage of "defined(__cpp_lib_optional)" for the
CLI11_STD_OPTIONAL path will cause that to fail. It's not blocking for me, but for other MSVC users it probably would be.

[henryiii]

What is the problem (I'm not as familiar with MSVC as I used to be)? Could you suggest a fix?

[jessey-git]

At first glance it could probably be removed entirely from the #if conditional check as it's sufficiently protected otherwise... I think.

Switching my project to /std:c++14 mode will properly skip past, while /std:c++17 and /std:c++latest will properly include (assuming the __cpp_lib_optional check is removed). I'm using MSVC 15.7 here but it should be good downwards to 15.3 I would think.

In any case, I don't believe "__cpp_lib_optional" is defined for any compiler? The closest I could find is "__cpp_lib_experimental_optional" per: http://en.cppreference.com/w/cpp/experimental/lib_extensions

[henryiii]

It's listed here: https://en.cppreference.com/w/cpp/experimental/feature_test

I know that __cpp_lib_experimental_optional is not always defined like it should be. Sometimes these headers are present but empty or partial. It might be okay to remove it, but not sure.

[jessey-git]

Weird, cppreference's own search pulls up nothing for it :)

If it is part of the feature test stuff, it will only be available for 15.8 due in a few months I guess: https://twitter.com/StephanTLavavej/status/996830461031677952

Well, as it stands, there is a workaround after your recent changes -- the MSVC user can explicitly #define CLI11_STD_OPTIONAL 1 if it's misdetected.

[henryiii]

It looks like feature tests are a bit more experimental than I thought. I'll have to revisit those tests (feel free to suggest or make a PR, otherwise it will likely be a week or so).

[jessey-git]

The new version seems to work fine in my project after your recent changes - thank you! You can close as necessary.