https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63801
--- Comment #2 from Ryan Schmidt <gcc at ryandesign dot com> --- Isn't the extern "C" coming from lines 32-38 of gcc/graphite-clast-to-gimple.c which in version 5-20141109 say: #if defined(__cplusplus) extern "C" { #endif #include <isl/val_gmp.h> #if defined(__cplusplus) } #endif The first 4 lines of /opt/local/include/isl/val_gmp.h are simply: #ifndef ISL_VAL_GMP_H #define ISL_VAL_GMP_H #include <gmp.h> Lines 31-36 of /opt/local/include/gmp.h (the first non-comment lines) are: #ifndef __GMP_H__ #if defined (__cplusplus) #include <iosfwd> /* for std::istream, std::ostream, std::string */ #include <cstdio> #endif