For old gcc versions BOOST_MP11_HAS_TEMPLATE_AUTO isn't set

[rolandschulz]

Old versions of gcc (7.1-8.2) don't define __cpp_nontype_template_parameter_auto but only __cpp_template_auto. Because of that BOOST_MP11_HAS_TEMPLATE_AUTO doesn't get set even though those versions support it. spirit checks both (https://github.com/boostorg/spirit/blob/59515f0e56aebdf958eadab30be99cac8872e723/include/boost/spirit/home/x3.hpp#L19). Would this make sense for mp11 too?

[pdimov]

Could be worth a try; looks like __cpp_template_auto is set for Clang down to 4.0, I wonder whether such an old compiler would be able to actually compile the things.

[pdimov]

Looks like GCC 7 has a pre-standard implementation of auto in template parameters - it retains the value, but not the type, which leads to odd things like 0L being treated as false. (Interestingly, this seems to be the case for MSVC 14.1 as well.)

So this leaves 8.1 and 8.2, and I wonder whether it's worth the bother, as I see 8.4 on ubuntu-20.04.

[Lastique]

There is now BOOST_NO_CXX17_AUTO_NONTYPE_TEMPLATE_PARAMS in Boost.Config that indicates support for this feature. It includes older compilers.