mingw-runtime has a system header called <io.h> (low-level UNIXish I/O routines). This conflicts with libgfortran/io/io.h when "io.h" is included, causing bootstrap to fail when libgfortran is built.
-- Summary: libgfortran system header conflict breaks bootstrap on MinGW Product: gcc Version: 4.0.0 Status: UNCONFIRMED Severity: normal Priority: P2 Component: fortran AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: aaronavay62 at aaronwl dot com CC: gcc-bugs at gcc dot gnu dot org GCC target triplet: i686-pc-mingw32 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18103