meson can't figure out the correct linker for flang

[andyfaff]

Describe the bug
When building the SciPy package with msvc and flang (installed using conda) meson can't detect which linker should be used for flang. @eli-schwartz

To Reproduce
This reproducer assumes that the scipy repo is already cloned.

conda create -n test python=3.10
conda activate test
mamba install clangdev flang lld
set cc=clang-cl
set cxx=clang-cl
pip install meson ninja cython numpy pybind11 pythran
meson build

Output Error message:

(test) H:\programming\scipy>meson build
The Meson build system
Version: 0.63.2
Source dir: \\filer\home\anz\programming\scipy
Build dir: \\filer\home\anz\programming\scipy\build
Build type: native build
Project name: SciPy
Project version: 1.10.0.dev0
C compiler for the host machine: clang-cl (clang-cl 11.0.1)
C linker for the host machine: lld-link lld-link 11.1.0
C++ compiler for the host machine: clang-cl (clang-cl 11.0.1)
C++ linker for the host machine: lld-link lld-link 11.1.0
Host machine cpu family: x86_64
Host machine cpu: x86_64
Compiler for C supports arguments -Wno-unused-but-set-variable: NO
Compiler for C supports arguments -Wno-unused-function: YES
Compiler for C supports arguments -Wno-conversion: YES
Compiler for C supports arguments -Wno-misleading-indentation: YES
Compiler for C supports arguments -Wno-incompatible-pointer-types: YES
Library m found: NO

\\filer\home\anz\programming\scipy\meson.build:55:0: ERROR: Unable to detect linker for compiler `flang -Wl,--version`
stdout:
stderr: flang: error: no input files

It seems that flang -Wl,--version does not furnish the details that meson would like.

If cl is used as the c-compiler then the problem still occurs:

(test) set cc=
(test) set cxx=
(test) H:\programming\scipy>meson build
The Meson build system
Version: 0.63.2
Source dir: \\filer\home\anz\programming\scipy
Build dir: \\filer\home\anz\programming\scipy\build
Build type: native build
Project name: SciPy
Project version: 1.10.0.dev0
C compiler for the host machine: cl (msvc 19.29.30145 "Microsoft (R) C/C++ Optimizing Compiler Version 19.29.30145 for x64")
C linker for the host machine: link link 14.29.30145.0
C++ compiler for the host machine: cl (msvc 19.29.30145 "Microsoft (R) C/C++ Optimizing Compiler Version 19.29.30145 for x64")
C++ linker for the host machine: link link 14.29.30145.0
Host machine cpu family: x86_64
Host machine cpu: x86_64
Compiler for C supports arguments -Wno-unused-but-set-variable: NO
Compiler for C supports arguments -Wno-unused-function: NO
Compiler for C supports arguments -Wno-conversion: NO
Compiler for C supports arguments -Wno-misleading-indentation: NO
Compiler for C supports arguments -Wno-incompatible-pointer-types: NO
Library m found: NO

\\filer\home\anz\programming\scipy\meson.build:55:0: ERROR: Unable to detect linker for compiler `flang -Wl,--version`
stdout:
stderr: flang: error: no input files

Expected behavior
meson should detect which linker is being used by flang.

system parameters
*This is a native build

• Windows 10
• Python 3.10 installed by conda
• meson version 0.63.2
• ninja version 1.10.2.git.kitware.jobserver-1

[rgommers]

Some more context, from scipy/scipy#16956 (comment):

[@eli-schwartz] Some linkers, like Windows link.exe, are run directly -- and Meson handles that in guess_win_linker().

[@rgommers] In the compiler detection logic, it seems like flang only occurs once, and it uses guess_nix_linker:

meson/mesonbuild/compilers/detect.py

Lines 815 to 818 in d285be7

	if 'flang' in out or 'clang' in out:
	linker = guess_nix_linker(env,
	compiler, FlangFortranCompiler, version, for_machine)
	return FlangFortranCompiler(

Perhaps the MSVC + Flang combo is unexpected there.

[h-vetinari]

I think I hit this, and solved it with some pointers from @eli-schwartz. I'l open a PR. Note though that the flang 11 you get from conda-forge is still the "classic" flang, not the new llvm-based one.