Clang-cl flags -Wmicrosoft-unqualified-friend warnings

[rubenvb]

In Boost 1.70 (and current master, but I don't have the line numbers handy), these lines spawn a warning:

unqualified friend declaration referring to type outside of the nearest enclosing namespace is a Microsoft extension; add a nested name specifier
map_view_iterator.hpp:
87, 138, 174, 215, 252, 293, 329

It seems to me that simply adding a ::boost::iterators:: qualification will fix this issue. See e.g. https://kod.acikkaynak.gov.tr/libreoffice/core/commit/aef6da021da33f099f99ed633814b8771ad4abda which seems to "fix" the warning locally by disabling the warning.

You can try Clang-cl by installing the CLang-cl options in the VS2019 installer, and e.g. running CMake with -T"ClangCL", or specifying the compiler manually using -DCMAKE_CXX_COMPILER=path/to/clang-cl.exe. You can also just use LLVM's version of clang-cl, it seems to be identical. Just make sure you have either VS2017 or VS2019 installed.