Use ar -L when available

As of 9.4, GHC on Windows may produce .o files that are in fact archive
files due to the lack of object merging support in the lld linker (see
GHC #21068). Consequently, we must ensure that `ar` merges input
archives' members, not add them as a single entry.

Author: bgamari

Cabal/src/Distribution/Simple/Compiler.hs

@@ -62,6 +62,7 @@ module Distribution.Simple.Compiler (
         profilingSupported,
         backpackSupported,
         arResponseFilesSupported,
+        arDashLSupported,
         libraryDynDirSupported,
         libraryVisibilitySupported,
 
@@ -365,6 +366,12 @@ libraryDynDirSupported comp = case compilerFlavor comp of
 arResponseFilesSupported :: Compiler -> Bool
 arResponseFilesSupported = ghcSupported "ar supports at file"
 
+-- | Does this compiler's "ar" command support llvm-ar's -L flag,
+-- which compels the archiver to add an input archive's members
+-- rather than adding the archive itself.
+arDashLSupported :: Compiler -> Bool
+arDashLSupported = ghcSupported "ar supports -L"
+
 -- | Does this compiler support Haskell program coverage?
 coverageSupported :: Compiler -> Bool
 coverageSupported comp =

Cabal/src/Distribution/Simple/Program/Ar.hs

@@ -24,7 +24,7 @@ import Distribution.Compat.Prelude
 import qualified Data.ByteString as BS
 import qualified Data.ByteString.Char8 as BS8
 import Distribution.Compat.CopyFile (filesEqual)
-import Distribution.Simple.Compiler (arResponseFilesSupported)
+import Distribution.Simple.Compiler (arResponseFilesSupported, arDashLSupported)
 import Distribution.Simple.LocalBuildInfo (LocalBuildInfo(..))
 import Distribution.Simple.Program
          ( ProgramInvocation, arProgram, requireProgram )
@@ -68,17 +68,25 @@ createArLibArchive verbosity lbi targetPath files = do
   --     do that. We have duplicates because of modules like "A.M" and "B.M"
   --     both make an object file "M.o" and ar does not consider the directory.
   --
+  --  -- llvm-ar, which GHC >=9.4 uses on Windows, supports a "L" modifier
+  --     in "q" mode which compels the archiver to add the members of an input
+  --     archive to the output, rather than the archive itself. This is
+  --     necessary as GHC may produce .o files that are actually archives. See
+  --     https://gitlab.haskell.org/ghc/ghc/-/issues/21068.
+  --
   -- Our solution is to use "ar r" in the simple case when one call is enough.
   -- When we need to call ar multiple times we use "ar q" and for the last
   -- call on OSX we use "ar qs" so that it'll make the index.
 
   let simpleArgs  = case hostOS of
              OSX -> ["-r", "-s"]
+             _ | dashLSupported -> ["-qL"]
              _   -> ["-r"]
 
       initialArgs = ["-q"]
       finalArgs   = case hostOS of
              OSX -> ["-q", "-s"]
+             _ | dashLSupported -> ["-qL"]
              _   -> ["-q"]
 
       extraArgs   = verbosityOpts verbosity ++ [tmpPath]
@@ -90,8 +98,10 @@ createArLibArchive verbosity lbi targetPath files = do
 
       oldVersionManualOverride =
         fromFlagOrDefault False $ configUseResponseFiles $ configFlags lbi
-      responseArgumentsNotSupported   =
+      responseArgumentsNotSupported =
         not (arResponseFilesSupported (compiler lbi))
+      dashLSupported =
+        arDashLSupported (compiler lbi)
 
       invokeWithResponesFile :: FilePath -> ProgramInvocation
       invokeWithResponesFile atFile =