define_if_constexpr_macro.hpp inclusion & config.hpp

[urnathan]

It seems that boost/multi_index/detail/define_if_constexpr_macro.hpp is included inside inline member functions (multi_index_container.hpp around line 370 for instance). define_if_constexpr_macro.hpp #includes <boost/config.h>. That's problematic.

It causes problems with clang implicit modules, if one tries to make boost/config.h (/and/ its entire subgraph) importable headers (and by implication C++20 header units).

But AFAICT, it's just plain wrong, because boost/config.hpp can expand to namespace-scope code on first inclusion -- config/detail/suffix.hpp contains typedefs for a 64bit int type and templates for missing min/max fns. So if define_if_constexpr_macro.hpp contains the first inclusion, things will go sadly wrong anyway. (I don't think this happens, because the users of define_if_constexpr_macro have already included config.hpp.

Either remove the #include <boost/config.hpp> from define_if_constexpr_macro.hpp or replace it with something like:

-#include <boost/config.hpp>
+#ifndef BOOST_CONFIG_HPP
+// We can't #include config.hpp ourselves as it can expand to
+// top-level source, and this file is intended for inclusion inside
+// functions (for instance).
+#error "#include <boost/config.hpp> missing"
+#endif

[joaquintides]

Thanks for spotting this problem! Fixed in 99f33de.