We could consider a slightly different approach. The common idiom in such cases is to bring a function into the scope with 'using' keyword and then call the function without explicitly defining the namespace. This way the ADL can kick in and search for a function in more places.
E.g. see:
https://github.com/boostorg/math/blob/develop/include/boost/math/special_functions/beta.hpp line 35 and 104
and for definition of BOOST_MATH_STD_USING
https://github.com/boostorg/math/blob/master/include/boost/math/tools/config.hpp line 255
or
https://github.com/boostorg/math/blob/develop/include/boost/math/special_functions/sinhc.hpp line 42