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Now that HEAD uses Clang/lld as its C toolchain, I decided to try it out on some code that links against the C++ standard library. Unfortunately, I wasn't able to get very far. In fact, I wasn't even able to get the text library to build after https://github.com/haskell/text/pull/429.

To reproduce:

1. Grab a nightly Windows bindist. I'm using a bindist at commit f2c08124.

2. Grab a nightly Windows build of cabal-install. I'm using the one here.

3. Clone the https://github.com/haskell/text repo and check out commit 89feee7:

   $ git clone https://github.com/haskell/text && cd text/
   $ git reset --hard 89feee7e8a3b7e55dc8544cd21ea5f908cc8aea9

4. Using the HEAD versions of cabal-install and GHC, build text and write a GHC environment file:

   $ <path-to-cabal>/cabal.exe build -w <path-to-ghc>/ghc-9.3.20220425/bin/ghc --allow-newer --write-ghc-environment-files=always --keep-going

5. Try compiling the following example program in the same directory and running it:

   module Main where
   
   import Data.Text
   
   main :: IO ()
   main = print $ pack "Hello, World!"

   $ $USERPROFILE/Software/ghc-9.3.20220425/bin/ghc Main.hs
   Loaded package environment from C:\Users\winferno\Documents\Hacking\Haskell\text\.ghc.environment.x86_64-mingw32-9.3.20220425
   [1 of 2] Compiling Main             ( Main.hs, Main.o )
   [2 of 2] Linking Main.exe
   
   $ ./Main.exe
   C:/Users/winferno/Documents/Hacking/Haskell/text/Main.exe: error while loading shared libraries: libc++.dll: cannot open shared object file: No such file or directory

   As this demonstrates, loading fails before the program can be run.

6. A similar failure can be observed when loading the program into GHCi:

   $ $USERPROFILE/Software/ghc-9.3.20220425/bin/ghci Main.hs
   WARNING: GHCi invoked via 'ghci.exe' in MinTTY consoles (e.g., Cygwin or MSYS)
            doesn't handle Ctrl-C well; use the 'ghcii.sh' shell wrapper instead
   Loaded package environment from C:\Users\winferno\Documents\Hacking\Haskell\text\.ghc.environment.x86_64-mingw32-9.3.20220425
   GHCi, version 9.3.20220425: https://www.haskell.org/ghc/  :? for help
   ghc-9.3.20220425.exe: Could not load `libc++.dll'. Reason: addDLL: libc++.dll or dependencies not loaded. (Win32 error 126)
   
   Loaded GHCi configuration from C:\Users\winferno\AppData\Roaming\ghc\ghci.conf
   Ok, one module loaded.
   > main
   ghc-9.3.20220425.exe:  | C:\Users\winferno\Software\ghc-9.3.20220425\mingw\lib\libc++abi.a: unknown symbol `_ZNSt3__119__libcpp_mutex_lockEPPv'
   ghc-9.3.20220425.exe:  | C:\Users\winferno\Software\ghc-9.3.20220425\mingw\lib\libc++abi.a: unknown symbol `_ZN10__cxxabiv130__aligned_malloc_with_fallbackEy'
   ghc-9.3.20220425.exe:  | C:\Users\winferno\Software\ghc-9.3.20220425\mingw\lib\libc++abi.a: unknown symbol `__cxa_begin_catch'
   ghc-9.3.20220425.exe:  | C:\Users\winferno\Software\ghc-9.3.20220425\mingw\lib\libc++abi.a: unknown symbol `_ZSt15get_new_handlerv'
   ghc-9.3.20220425.exe:  | C:\Users\winferno\Documents\Hacking\Haskell\text\dist-newstyle\build\x86_64-windows\ghc-9.3.20220425\text-2.0\build\libHStext-2.0-inplace.a: unknown symbol `_ZdlPv'
   ghc-9.3.20220425.exe:  | C:\Users\winferno\Documents\Hacking\Haskell\text\dist-newstyle\build\x86_64-windows\ghc-9.3.20220425\text-2.0\build\libHStext-2.0-inplace.a: unknown symbol `_ZN7simdutf13validate_utf8EPKcy'
   ghc-9.3.20220425.exe:  | C:\Users\winferno\Documents\Hacking\Haskell\text\dist-newstyle\build\x86_64-windows\ghc-9.3.20220425\text-2.0\build\libHStext-2.0-inplace.a: unknown symbol `_hs_text_is_valid_utf8'
   ghc-9.3.20220425.exe:  | C:\Users\winferno\Documents\Hacking\Haskell\text\dist-newstyle\build\x86_64-windows\ghc-9.3.20220425\text-2.0\build\libHStext-2.0-inplace.a: unknown symbol `textzm2zi0zminplace_DataziTextziEncoding_zdwdecodeUtf8With_info'
   ghc-9.3.20220425.exe:  | C:\Users\winferno\Documents\Hacking\Haskell\text\Main.o: unknown symbol `textzm2zi0zminplace_DataziText_pack_closure'
   ghc-9.3.20220425.exe: Could not load Object Code C:\Users\winferno\Documents\Hacking\Haskell\text\Main.o.

---

Some further observations:

• One hypothesis for why this is failing is because text is trying to link against dynamic libraries in this line in its .cabal file:

          extra-libraries: c++ c++abi

  If I change it in an effort to force GHC to link against the static versions of these libraries:

          extra-libraries: :libc++.a :libc++abi.a

  Then I am able to compile the example program and run it with ./Main.exe. However, I am still unable to run it with GHCi:

  $ <path-to-ghc>/ghc-9.3.20220425/bin/ghci Main.hs
  WARNING: GHCi invoked via 'ghci.exe' in MinTTY consoles (e.g., Cygwin or MSYS)
           doesn't handle Ctrl-C well; use the 'ghcii.sh' shell wrapper instead
  Loaded package environment from C:\Users\winferno\Documents\Hacking\Haskell\text\.ghc.environment.x86_64-mingw32-9.3.20220425
  GHCi, version 9.3.20220425: https://www.haskell.org/ghc/  :? for help
  <command line>: addDLL: :libc++.a or dependencies not loaded. (Win32 error 126)

• Adding ghc-options: -static to text.cabal makes no difference.

• I did not have mingw-w64-x86_64-libc++ or mingw-w64-x86_64-libc++abi installed in the steps above. If I install them and then repeat the steps above, then I am able to compile the example program and run it with ./Main.exe. However, I am still unable to run it with GHCi:

  $ $USERPROFILE/Software/ghc-9.3.20220425/bin/ghci Main.hs
  WARNING: GHCi invoked via 'ghci.exe' in MinTTY consoles (e.g., Cygwin or MSYS)
           doesn't handle Ctrl-C well; use the 'ghcii.sh' shell wrapper instead
  Loaded package environment from C:\Users\winferno\Documents\Hacking\Haskell\text\.ghc.environment.x86_64-mingw32-9.3.20220425
  GHCi, version 9.3.20220425: https://www.haskell.org/ghc/  :? for help
  Loaded GHCi configuration from C:\Users\winferno\AppData\Roaming\ghc\ghci.conf
  Ok, one module loaded.
  > main
  ghc-9.3.20220425.exe:  | C:\Users\winferno\Software\ghc-9.3.20220425\mingw\lib\libc++abi.a: unknown symbol `_ZdlPv'
  ghc-9.3.20220425.exe:  | C:\Users\winferno\Software\ghc-9.3.20220425\mingw\lib\libc++abi.a: unknown symbol `_ZTISt13bad_exception'
  ghc-9.3.20220425.exe:  | C:\Users\winferno\Software\ghc-9.3.20220425\mingw\lib\libc++abi.a: unknown symbol `__gxx_personality_seh0'
  ghc-9.3.20220425.exe:  | C:\Users\winferno\Software\ghc-9.3.20220425\mingw\lib\libc++abi.a: unknown symbol `_ZN10__cxxabiv130__aligned_malloc_with_fallbackEy'
  ghc-9.3.20220425.exe:  | C:\Users\winferno\Software\ghc-9.3.20220425\mingw\lib\libc++abi.a: unknown symbol `__cxa_begin_catch'
  ghc-9.3.20220425.exe:  | C:\Users\winferno\Software\ghc-9.3.20220425\mingw\lib\libc++abi.a: unknown symbol `_ZSt15get_new_handlerv'
  ghc-9.3.20220425.exe:  | C:\Users\winferno\Documents\Hacking\Haskell\text\dist-newstyle\build\x86_64-windows\ghc-9.3.20220425\text-2.0\build\libHStext-2.0-inplace.a: unknown symbol `_ZdlPv'
  ghc-9.3.20220425.exe:  | C:\Users\winferno\Documents\Hacking\Haskell\text\dist-newstyle\build\x86_64-windows\ghc-9.3.20220425\text-2.0\build\libHStext-2.0-inplace.a: unknown symbol `_ZN7simdutf13validate_utf8EPKcy'
  ghc-9.3.20220425.exe:  | C:\Users\winferno\Documents\Hacking\Haskell\text\dist-newstyle\build\x86_64-windows\ghc-9.3.20220425\text-2.0\build\libHStext-2.0-inplace.a: unknown symbol `_hs_text_is_valid_utf8'
  ghc-9.3.20220425.exe:  | C:\Users\winferno\Documents\Hacking\Haskell\text\dist-newstyle\build\x86_64-windows\ghc-9.3.20220425\text-2.0\build\libHStext-2.0-inplace.a: unknown symbol `textzm2zi0zminplace_DataziTextziEncoding_zdwdecodeUtf8With_info'
  ghc-9.3.20220425.exe:  | C:\Users\winferno\Documents\Hacking\Haskell\text\Main.o: unknown symbol `textzm2zi0zminplace_DataziText_pack_closure'
  ghc-9.3.20220425.exe: Could not load Object Code C:\Users\winferno\Documents\Hacking\Haskell\text\Main.o.

• Somehow, the Windows CI on this MR, which bumps the text submodule to include 89feee7, compiles and passes the test suite without issue (example). I wonder if there is some configuration difference between the Windows CI runner and my own local machine.