Build failure with LLVM flang on macOS/Darwin — ltmain.sh flang detection fails with full compiler paths

[Matthew Mehrtens]

Hi,

I'm getting a build failure when building Open MPI 5.0.9 with LLVM compilers on macOS (Apple Silicon). Configure succeeds, but make fails when linking:

```
flang-21: error: unknown argument: '-compatibility_version'
flang-21: error: unknown argument: '-current_version'
flang-21: error: no such file or directory: '81'
flang-21: error: no such file or directory: '81.3'
make[3]: *** [libmpi_usempif08.la] Error 1

```

I believe this error comes from config/ltmain_flang_darwin.diff. The patch has three hunks that add flang-specific handling for Darwin linker flags. Hunks 1 and 3 match against $CC directly using `case $CC in flang*)`, which fails when the Fortran compiler is specified as a full path (e.g., /opt/homebrew/opt/flang/bin/flang). Hunk 2 correctly uses `func_cc_basename $CC` before matching, so it doesn't have this problem.

Environment:

• macOS 26.2 (Darwin 25.2.0), aarch64 (Apple M1 Pro)
  
• Homebrew clang/clang++/flang 21.1.8
  
• Open MPI 5.0.9

Configure command:

```

$ ./configure \
      CC=/opt/homebrew/opt/llvm/bin/clang \
      CPPFLAGS=-I/opt/homebrew/opt/llvm/include \
      CXX=/opt/homebrew/opt/llvm/bin/clang++ \
      CXXCPPFLAGS=-I/opt/homebrew/opt/llvm/include \
      FC=flang \
      LDFLAGS="-L/opt/homebrew/opt/llvm/lib -L/opt/homebrew/opt/llvm/lib/c++ -L/opt/homebrew/opt/llvm/lib/unwind -lunwind" \
      --with-hwloc=/opt/homebrew/opt/hwloc \
      --with-libevent=/opt/homebrew/opt/libevent \
      --with-pmix=/opt/homebrew/opt/pmix

```

Build output: https://www.icloud.com/iclouddrive/00dSkBghSZdP0-MRsCPN7M7pw#ompi-output (let me know if you cannot access this, and I'll resubmit using a different service)

Workaround: Use `FC=flang` (bare name) instead of the full path, with /opt/homebrew/opt/flang/bin in $PATH.

Suggested fix: Update hunks 1 and 3 in config/ltmain_flang_darwin.diff to use `func_cc_basename $CC` and `$func_cc_basename_result` instead of matching $CC directly, consistent with hunk 2.

If this is indeed a bug and not intentional, I'd be happy to submit an issue/PR on GitHub if you can advise on the preferred approach. I wasn't sure whether to regenerate the diff against a specific libtool version or patch it directly.

Thanks,
Matthew Mehrtens

[turnout4...@icloud.com]

Hi,

Bumping this thread. Is this the correct list to post this on?

Best regards,

Matthew Mehrtens

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[George Bosilca]

I would create a github issue at https://github.com/open-mpi/ompi. And feel free to propose a PR, this looks like a bug to me.

  George.

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[Matthew Mehrtens]

Thanks to @billsacks for opening a GitHub issue and PRs (main and v5.0.x) for this bug.

I’ve been meaning to get around to this but have been busy with school. Cheers all!

Best regards,

Matthew Mehrtens