Working with someone else's code here. It compiles just fine with gfortran. Under Portland Group, though, I get an error:
pgf90 -DsysLinux -DcompPGF90 -I/home/cables/GITM/share/Library/src -c -r8 -fast ModUtilities.F90
PGF90-S-0084-Illegal use of symbol mpi_wtime - not public entity of module (ModUtilities.F90: 419)
0 inform, 0 warnings, 1 severes, 0 fatal for sleep
The offending line looks like:
use ModMpi, ONLY : MPI_wtime
(There's obviously a lot of MPI stuff going on here, but I don't think that's the point.) So I go to the source code for ModMpi, which is ModMpi.f90, where I see no reference to MPI_WTIME, but I see:
use ModMpiInterfaces
So finally, I go to the source for ModMpiInterface and I find the line:
public:: mpi_wtime
OK, I was able to get a compile from PGI by editing ModMpi.f90 and declaring mpi_wtime to be public. But still, I wonder: Why did gfortran assume (apparently) that mpi_wtime was public, but PGI had to be told this explicitly? Why does PGI not assume that the original public declaration holds throughout the "use chain"?
I presume that one behavior or the other is closer to the Fortran standard. Which would that be?
For exactly the same Fortran source code (as opposed to some sort of MPI library) compiler behaviour should be the same here.
Whether or not an entity is a public entity of a module is specific to each module that defines or accesses (via USE) that entity. Module A might declare "something" and specify that it is public, module B might USE module A and then specify that same "something" is then private. Any code using module A will be able to access "something", any code only using module B will not.
The default accessibility of things declared in a module is PUBLIC, but that default can be changed by a PRIVATE statement (one without any following identifiers). If such a private statement appeared, you would see the behaviour you describe with the PGI compiler.
Implicit typing (i.e. from source code without IMPLICIT NONE) can also confuse things here.