Document gfortran compiler error and only enforce standard on newer versions

Author: sethrj

.travis.yml

@@ -13,7 +13,6 @@ matrix:
   include:
       - os: linux
         env: FLIBCPP_DEV=ON GENERATOR=ninja
-             FLIBCPP_FORTRAN_STD=f2003
         addons:
           apt:
             packages:
@@ -22,18 +21,20 @@ matrix:
             - valgrind
       - os: linux
         env: FLIBCPP_DEV=OFF GENERATOR=make
-             FLIBCPP_FORTRAN_STD=f2008
+             FLIBCPP_FORTRAN_STD=f2003
         addons:
           apt:
             packages:
-            - gfortran
+            - gcc-8
+            - gfortran-8
       - os: linux
         env: FLIBCPP_DEV=OFF GENERATOR=make
              FLIBCPP_FORTRAN_STD=f2008
         addons:
           apt:
             packages:
-            - gfortran
+            - gcc-9
+            - gfortran-9
 
 # Build phases
 before_install:

CMakeLists.txt

@@ -166,7 +166,7 @@ function(swig_fortran_add_module name)
       cxx_std_11
   )
 
-  if (CMAKE_Fortran_COMPILER_ID STREQUAL "GNU")
+  if (FLIBCPP_FORTRAN_STD AND CMAKE_Fortran_COMPILER_ID STREQUAL "GNU")
     # Compile Fortran code with given standard
     target_compile_options(${name}
       PUBLIC $<$<COMPILE_LANGUAGE:Fortran>:-std=${FLIBCPP_FORTRAN_STD}>

doc/examples.rst

@@ -38,8 +38,8 @@ applications that operate on large chunks of data. This example demonstrates
 the generic version of the :ref:`modules_algorithm_argsort` subroutine by
 sorting a native Fortran array of native Fortran types using a native Fortran
 subroutine. The only C interaction needed is to create C pointers to the
-Fortran array entries and to provide a C-bound comparator that converts those
-pointers back to native Fortran pointers.
+Fortran array entries and to provide a C-bound comparator that
+converts those pointers back to native Fortran pointers. [#c_f_pointer]_
 
 .. literalinclude:: ../example/sort_generic.f90
    :linenos:
@@ -60,6 +60,27 @@ provides procedures to:
 .. literalinclude:: ../example/example_utils.f90
    :linenos:
 
+
+.. rubric:: Footnotes
+
+.. [#c_f_pointer] Older versions of Gfortran (before GCC-8) fail to compile the
+   generic sort example because of a bug that incorrectly claims that taking
+   the C pointer of a scalar Fortran value is a violation of the standard:
+
+   .. code-block:: none
+
+      ../example/sort_generic.f90:84:38:
+
+           call c_f_pointer(cptr=rcptr, fptr=rptr)
+                                            1
+      Error: TS 29113/TS 18508: Noninteroperable array FPTR at (1) to
+      C_F_POINTER: Expression is a noninteroperable derived type
+
+   See `this bug report`_ for more details.
+
+   .. _this bug report: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84924
+
+
 .. ############################################################################
 .. end of doc/examples.rst
 .. ############################################################################