https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67068
--- Comment #6 from Dominique d'Humieres <dominiq at lps dot ens.fr> --- For openmpi-1.10.0 and trunk (6.0) compiling examples/hello_mpifh.f with -freal-4-real-8 gives mpif-sizeof.h:575:72: Error: Ambiguous interfaces 'mpi_sizeof_real64_r7' and 'mpi_sizeof_real32_r7' in generic interface 'mpi_sizeof' at (1) mpif-sizeof.h:1139:72: Error: Ambiguous interfaces 'pmpi_sizeof_real64_r7' and 'pmpi_sizeof_real32_r7' in generic interface 'pmpi_sizeof' at (1) If I compile with -fdefault-real-8 with/without -fdefault-double-8, I don't get the error. The (a) main difference between -fdefault-double-8 and -freal-4-real-8 is that the former converts only default REAL (not the variables defined as REAL(4)), while the later converts all REAL to REAL(8). Anyway mixing -fdefault-double-8 and -freal-4-real-8 is bad (as in forbidden). IMO this PR should be closed as INVALID.
