The sun c++ compiler doesn't issue a warning about comparison between signed and unsigned integer expressions. The attached patch makes mpl::print produce "implicit int is not supported" warning. With the patch applied the compiler output looks like this: $ CC +w -c print.cpp "/home/dgoncharov/boost/trunk/boost/mpl/print.hpp", line 55: Warning: Implicit int is not supported in C++. "print.cpp", line 3: Where: While specializing "boost::mpl::print<int>". "print.cpp", line 3: Where: Specialized in non-template code. 1 Warning(s) detected. Br, Dmitry