GHC 9.4+: unused '--unwindlib=libunwind' argument warning when compiling C++ code on Windows
Attempting to compile any project that uses C++ code on GHC 9.4 or later on Windows results in a warning that did not appear in previous versions of Windows. Here is a minimal example:
// hello.cpp
#include <iostream>
extern "C"
void hello() {
std::cout << "Hello, World!" << std::endl;
}
-- Main.hs
{-# LANGUAGE ForeignFunctionInterface #-}
module Main (main) where
foreign import ccall unsafe "hello" cpp_hello :: IO ()
main :: IO ()
main = cpp_hello
On GHC 9.2.3, this compiles without warnings:
$ ghc-9.2.3 -fforce-recomp Main.hs hello.cpp -lstdc++
[1 of 1] Compiling Main ( Main.hs, Main.o )
Linking Main.exe ...
On GHC 9.4+, however:
$ ../../../Software/ghc-9.5.20220527/bin/ghc -fforce-recomp Main.hs hello.cpp -lc++ -lc++abi
clang++: warning: argument unused during compilation: '--unwindlib=libunwind' [-Wunused-command-line-argument]
[1 of 2] Compiling Main ( Main.hs, Main.o )
[2 of 2] Linking Main.exe
Note the '--unwindlib=libunwind'
warning, which did not appear previously. This likely arises from this line. I'm unclear if clang++
supports --unwindlib
.