SQUASH ME

Add text about Fortran ABI issues.  Please squash this commit into the
larger docs commit about ABI issues.

Signed-off-by: Jeff Squyres <[email protected]>

docs/building-apps/abi-compatibility.rst

@@ -11,8 +11,43 @@ application compiled with Open MPI 4.1 can be executed with Open MPI
 There are however a few scenarios where an application compiled with
 Open MPI 4.1 might not execute correctly with Open MPI 5.0.
 
-- There were some minor changes in the Fortran Interfaces (Jeff, need
-  your help what to say here).
+- Fortran compilers provide varying degrees of ABI guarantees between
+  their releases.  As such, Open MPI can only provide ABI guarantees
+  with MPI applications that use the Fortran MPI bindings when all
+  three entities |mdash| Open MPI v4.x, Open MPI |ompi_series|, and
+  the MPI application |mdash| were built with exactly the same Fortran
+  compiler and environment.
+
+  If these conditions are met, Open MPI's ABI guarantees between the
+  Open MPI v4.x and |ompi_series| series are in effect, with the
+  exception of the following cases:
+
+  * When using the Fortran ``mpi`` module bindings with GCC compiler
+    versions prior to v4.8, non-compliant Fortran interfaces for
+    ``MPI_Comm_spawn_multiple``, ``MPI_Testall``, ``MPI_Testsome``,
+    ``MPI_Waitall``, and ``MPI_Waitsome`` were removed starting with
+    Open MPI v5.0.0.
+
+  * When using the Fortran ``mpi`` module with modern Fortran
+    compilers (e.g., GNU Fortran >= v4.9), Open MPI v5.0.0 removed the
+    names from the MPI interfaces when there is only a single
+    subroutine in the interface, and that subroutine name exactly
+    matches the iterface name.  This change is likely to make Open MPI
+    |ompi_series|'s ``mpi`` module bindings *less* restrictive than
+    Open MPI v4.x, but it *may* also have ABI implications, depending
+    on your Fortran compiler.
+
+    `See this git commit message
+    <https://github.com/open-mpi/ompi/commit/f34782fe0c493963ec44bcf7dde9d94b88a7ea91>`_
+    for more details.
+
+    .. important:: This is likely a compiler-specific issue, and may
+                   need to be tested in your environment.
+
+  * When using the Fortran ``mpi_f08`` module bindings in an
+    environment where a Fortran ``INTEGER`` is 8 bytes but a C ``int``
+    is 4 bytes, the size of a ``Type(MPI_Status)`` was increased
+    starting with Open MPI v5.0.0.
 
 - Open MPI 5.0 removed support for the C++ bindings. If an application
   was using the deprecated and now removed C++ bindings, it will not