Unable to initialize static set of strings inside templatized class in CLang, works fine in GCC - string

Question

I'm using Boost config_file_iterator to parse a parameters file for different modules in the standard Boost way:
[Module 1 name]
M1_Key1=value1
[Module 2 name]
M2_Key1=value2
For that, Boost has the nice feature to accept a std::set containing all the valid keys, to throw an exception in case a non-valid one is passed.
boost::program_options::detail::config_file_iterator cfgIt(configStream, ALLOWED_PARAMS);
In my code, I've a set of modules, each one with its own set of parameters, In each one of the module header I have (as non-member variables):
module1.h:
static const std::string MODULE1_NAME = "Module 1 name";
static const std::string MODULE1_KEY1 = "M1_Key1";
module2.h:
static const std::string MODULE2_NAME = "Module 2 name";
static const std::string MODULE2_KEY1 = "M2_Key1";
In the header file for the parameters parser:
class ParametersParser
{
static const std::set<std::string> ALLOWED_PARAMS;
}
Finally in the source file of that parameter parser groups all the allowed values for the config iterator from the different modules:
#include "module1.h"
#include "module2.h"
const std::set<std::string> ParametersParser::ALLOWED_PARAMS =
{{
MODULE1_NAME + std::string(".") + MODULE1_KEY1,
MODULE2_NAME + std::string(".") + MODULE2_KEY1
}};
since Boost expects that as the allowed parameters set, a set of string of the way "Module X Name.MX_KeyY", and that worked fine.
However, because of other reasons, I've templatized the class that parses the file, so now my set of strings is declared also in the same header file, as:
#include "module1.h"
#include "module2.h"
/* Class definition */
template <class T>
class ParametersParser
{
static const std::set<std::string> ALLOWED_PARAMS;
};
/* Static member initialization */
template <class T>
const std::set<std::string> ParametersParser<T>::ALLOWED_PARAMS =
{{
MODULE1_NAME + std::string(".") + MODULE1_KEY1,
MODULE2_NAME + std::string(".") + MODULE2_KEY1
}};
And here is where the problem arises: This compiles in both gcc 4.8.5 (CentOS 7.5 vanilla) and AppleClang 9.1.0.9020039 (MacOSX 10.3), but in MacOSX, the resulting set only contains a ".". It's like the static strings defined in the modules headers (MODULE1_NAME, MODULE1_KEY1, etc) are empty!!! In gcc/CentOS, it works fine. Unfortunately, I couldn't compile on gcc for MacOSX (It complains about unresolved dependencies with Boost), while CLang on CentOS (Apple LLVM version 9.1.0 (clang-902.0.39.2)) works fine too.
Do you have any clues what am I doing wrong? Is it something about using strings as static variables? I also tried declaring the set using the new initializer list syntax, const std::set<std::string> ParametersParser<T>::ALLOWED_PARAMS{{...}}, but didn't work either.
Thanks a lot for your help.