[flang-commits] [clang] [flang] [flang][driver] Make -stdlib= option visible to flang and silently ignored by it (PR #110598)

[Michael Kruse via flang-commits]

Meinersbur wrote:

While the FortranRuntime is written in C++, it is ensured that it does not depend on libc++.so/libstdc++.so ([regression test](https://github.com/llvm/llvm-project/blob/main/flang/test/Runtime/no-cpp-dep.c)). Hence what C++ runtime the user links into their executable shouldn't concern the Fortran part.

When mixing Fortran and C++ code with Fortran code in the same add_library/add_executable, will use the C++ linker to link all the object files together (just as with C and C++). That might be the expected thing to do. That means it will care for linking the correct C++ library, but leave out the Fortran runtime library, and the user has to add `-lFortranRuntime` manually. If I understand correct, with this patch `flang-new -stdlib=libc++` will link the C++ and Fortran standard libraries? Unfortunately CMake does not know that.

Note that the test will fail when `LLVM_DEFAULT_TARGET_TRIPLE` is set to anything else than the host platform. Because of this. Clang tests never invoke the linker, but check the output of `-###`. However, Flang already has tests with the same issue.

https://github.com/llvm/llvm-project/pull/110598