gcc-13 warning: unknown pragma regarding -Wundefined-var-template

[anarthal]

detail/fake_object.hpp emits the following warning under gcc 13.1 (Linux):

./boost/pfr/detail/fake_object.hpp:23:35: error: unknown option after ‘#pragma GCC diagnostic’ kind [-Werror=pragmas]
   23 | #   pragma GCC diagnostic ignored "-Wundefined-var-template"

You need to build with -Wall to get it. Actually, any code with the line # pragma GCC diagnostic ignored "-Wundefined-var-template" emits such a warning. gcc-11 also emits the same warning. I don't have an exhaustive list of compilers, but it looks like GCC is not understanding this line.

This has been introduced in Boost 1.85. I build with -Wall -Wextra -Werror, so these are a little bit annoying.