Removal of global placeholders causes multiple software to fail even when BOOST_BIND_GLOBAL_PLACEHOLDERS is set

Multiple software suffered from boost global placeholders removal causing "_1 was not declared in this scope" like compiler errors. The backward compatibility was supposed to be addressed by BOOST_BIND_GLOBAL_PLACEHOLDERS, it looked like you define BOOST_BIND_GLOBAL_PLACEHOLDERS and then magically bind.hpp header is pulled in everything boost uses returning the old logic back.

But that macros is just a plug that does nothing but:

#if !defined(BOOST_BIND_GLOBAL_PLACEHOLDERS)

BOOST_PRAGMA_MESSAGE(
"The practice of declaring the Bind placeholders (_1, _2, ...) "
"in the global namespace is deprecated. Please use "
" + using namespace boost::placeholders, "
"or define BOOST_BIND_GLOBAL_PLACEHOLDERS to retain the current behavior."
)

#endif

defining or not defining it just affects the message, but not the logic.

When BOOST_BIND_GLOBAL_PLACEHOLDERS is defined the old logic of registering global placeholders should be back with everything using any boost header for at least 2-3 next years when all the developers patch their software.

Pulling _1 in the global scope might be the bad practice but that practice was set by boost and used widely by many developers

Start by tracing how boost/bind/bind.hpp and bind.hpp handle BOOST_BIND_GLOBAL_PLACEHOLDERS, then compare the macro's documented compatibility behavior with the current placeholder declarations. Done means defining the macro restores global _1, _2 and related placeholders for code including Boost headers, while leaving the deprecation message behavior unchanged when it is not defined.

Written by the indexing model from the issue text.