Can't directly compare optional<int> to nullopt under GCC

[tonyelewis]

The following code compiles under Clang but not under GCC:

#include <boost/test/unit_test.hpp>
#include <optional>

BOOST_TEST_DONT_PRINT_LOG_VALUE( ::std::nullopt_t     )
BOOST_TEST_DONT_PRINT_LOG_VALUE( ::std::optional<int> )

BOOST_AUTO_TEST_CASE(y) {
    BOOST_TEST( ::std::make_optional( 5 ) == ::std::nullopt );
}

I suspect that there's an issue with GCC's std::is_constructible needlessly explicitly instantiating boost::test_tools::assertion::expression_base's operator bool() and I've raised 87093 about that.

However, (a) the GCC devs may well reject that issue and (b) I suspect it's workaround-able either way and it'd be really nice to be able to express this directly. Please can you look into this?

Thanks for all your work on this library.

[raffienficiaud]

Thank you for those kind word!

So ... the patch is in GCC now, what do you want me to do ? :-) I can create a dedicated comparison for std::optional.

[tonyelewis]

I was wondering whether there's a way to achieve whatever the relevant piece of metaprogramming is trying to achieve with out stepping on this particular GCC issue.

In particular, this GCC issue appears to be having trouble with std::is_constructible<int, boost::test_tools::assertion::value_expr<std::optional<int> >&&> in that it tries to instantiate an operator bool being considered for the conversion.

Is there a way to perform the checks that get to this problem without invoking this issue?