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A follow-up from Discourse. I've attached a tarball of the CMakeLists/ for a tiny project that demonstrates the problem: CMakeFiles.tar.gz

FortranCInterface_VERIFY isn’t working on my Mac after an OS and compiler updates. I’m running on macOS 11 with gcc-10 from Homebrew and cmake 3.18. Here are some of the relevant errors from stdout:

-- Detecting Fortran/C Interface
Failed to compile
-- Verifying Fortran/C Compiler Compatibility
Failed to compile
CMake Warning (dev) at /usr/local/Cellar/cmake/3.18.4/share/cmake/Modules/FortranCInterface.cmake:309 (message):
  No FortranCInterface mangling known for VerifyFortran
Call Stack (most recent call first):
  /usr/local/Cellar/cmake/3.18.4/share/cmake/Modules/FortranCInterface/Verify/CMakeLists.txt:16 (FortranCInterface_HEADER)
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Verifying Fortran/C Compiler Compatibility - Failed
CMake Error at /usr/local/Cellar/cmake/3.18.4/share/cmake/Modules/FortranCInterface.cmake:383 (message):
  The Fortran compiler:

    /usr/local/bin/gfortran-10

  and the C compiler:

    /usr/local/bin/gcc-10

  failed to compile a simple test project using both languages.  The output
  was:

****** A BUNCH OF STUFF ****

    /usr/local/bin/gcc-10 -O3 -DNDEBUG -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk -Wl,-search_paths_first -Wl,-headerpad_max_install_names CMakeFiles/VerifyFortranC.dir/main.c.o CMakeFiles/VerifyFortranC.dir/VerifyC.c.o -o VerifyFortranC  libVerifyFortran.a -lgfortran -lquadmath -lm
    ld: warning: ignoring file libVerifyFortran.a, building for macOS-x86_64 but attempting to link with file built for macOS-x86_64
    Undefined symbols for architecture x86_64:
      "_VerifyFortran", referenced from:
          _main in main.c.o
    ld: symbol(s) not found for architecture x86_64
    collect2: error: ld returned 1 exit status

I did some detective work as saw that the name-mangling was indeed wrong.

$ nm CMakeFiles/FortranCInterface/VerifyC/libVerifyFortran.a
VerifyFortran.f.o:
00000000000000d0 r EH_frame1
                 U __gfortran_st_write
                 U __gfortran_st_write_done
                 U __gfortran_transfer_character_write
0000000000000000 T _verifyfortran_
0000000000000058 r lC0
00000000000000c8 r lC1
00000000000000b8 r lC2

$ nm CMakeFiles/FortranCInterface/VerifyC/CMakeFiles/VerifyFortranC.dir/main.c.o
0000000000000018 r EH_frame1
                 U _VerifyC
                 U _VerifyFortran
0000000000000000 T _main

$ cat ./CMakeFiles/FortranCInterface/VerifyC/VerifyFortran.h
#ifndef FortranCInterface_HEADER_INCLUDED
#define FortranCInterface_HEADER_INCLUDED

/*--------------------------------------------------------------------------*/
/* Mangle some symbols automatically.                                       */

#endif