Hi, when compiling GiNaC (latest version from git master) with MSVC (19.29.30139 for x64) it stumbles over the following code in lst.h/lst.cpp: lst.h line 35: template<> registered_class_info lst::reg_info; lst.cpp line 28: template <> GINAC_IMPLEMENT_REGISTERED_CLASS_OPT_T(lst, basic, print_func<print_context>(&lst::do_print). print_func<print_tree>(&lst::do_print_tree)) and says: lst.cpp(28): error C2766: explicit specialization; 'reg_info' has already been defined exprseq.h/.cpp has the same issue. A solution seems to be to remove the code in lst.cpp and replace the code in lst.h with it. That is lst.h: template <> GINAC_IMPLEMENT_REGISTERED_CLASS_OPT_T(lst, basic, print_func<print_context>(&lst::do_print). print_func<print_tree>(&lst::do_print_tree)) This compiles fine and all checks pass, but it does seem wrong to have the GINAC_IMPLEMENT_REGISTERED_CLASS_OPT_T in a header file. What is the right way to deal with this error? Thanks, Jan Rheinländer