Merge pull request #11763 from zlonast/zlonast/pgmcxx-option

Adding an explicit pgmcxx flag for interaction with GHC

Author: mergify[bot]

Cabal/src/Distribution/Simple/GHC/Internal.hs

@@ -361,8 +361,16 @@ splitCandCxxOptions
   -> GhcOptions
 splitCandCxxOptions source verbosity lbi bi clbi odir filename = case source of
   CxxProgram ->
-    setCcProgram $ setCcOptions $ sourcesGhcOptions verbosity lbi bi clbi odir filename
+    -- For C++ sources: reset ccOptions for GHC < 8.10, because on those
+    -- old GHCs there's no -optcxx flag — all options go through -optc.
+    -- Without this reset, C-specific flags (ccOptions) would leak into
+    -- C++ compilation via -optc, which is wrong.
+    setGppProgram $ setCcOptions $ sourcesGhcOptions verbosity lbi bi clbi odir filename
   CcProgram ->
+    -- For C sources: reset cxxOptions for GHC < 8.10, because on those
+    -- old GHCs there's no -optcxx flag — all options go through -optc.
+    -- Without this reset, C++-specific flags (cxxOptions) would leak into
+    -- C compilation via -optc, which is wrong.
     setCcProgram $ setCxxOptions $ sourcesGhcOptions verbosity lbi bi clbi odir filename
   where
     setCcOptions xxx =
@@ -406,6 +414,29 @@ splitCandCxxOptions source verbosity lbi bi clbi odir filename = case source of
           -- see example in cabal-testsuite/PackageTests/FFI/ForeignOptsPgmc
           ghcOptCcProgram = maybeToFlag $ programPath <$> lookupProgram gccProgram (withPrograms lbi)
         }
+    setGppProgram xxx =
+      xxx
+        { -- We explicitly pass the C++ compiler via -pgmcxx because GHC >= 9.4
+          -- does not automatically detect it from the toolchain. Without this GHC
+          -- falls back to a gcc as a C++ compiler.
+          -- Related: #11805
+          ghcOptGppProgram =
+            ghcOptionsSince
+              (mkVersion [9, 4])
+              (compiler lbi)
+              (maybeToFlag $ programPath <$> lookupProgram gppProgram (withPrograms lbi))
+        , -- For GHC < 9.4, which does not support -pgmcxx, we fall back to
+          -- passing the C++ compiler via -pgmc. This ensures C++ sources are
+          -- compiled with the correct compiler even on older GHCs.
+          -- Note: we use gppProgram (C++ compiler), NOT gccProgram (C compiler),
+          -- because this path is specifically for C++ source files.
+          -- Related: #11805
+          ghcOptCcProgram =
+            ghcOptionsBefore
+              (mkVersion [9, 4])
+              (compiler lbi)
+              (maybeToFlag $ programPath <$> lookupProgram gppProgram (withPrograms lbi))
+        }
 
 sourcesGhcOptions
   :: VerbosityLevel
@@ -454,6 +485,15 @@ ghcOptionsSince ver comp defOptions =
       | otherwise -> mempty
     Nothing -> mempty
 
+-- Applies options only if the GHC version is less than the given one.
+ghcOptionsBefore :: Monoid a => Version -> Compiler -> a -> a
+ghcOptionsBefore ver comp defOptions =
+  case compilerCompatVersion GHC comp of
+    Just v
+      | v < ver -> defOptions
+      | otherwise -> mempty
+    Nothing -> mempty
+
 componentGhcOptions
   :: VerbosityLevel
   -> LocalBuildInfo
@@ -552,6 +592,12 @@ linkGhcOptions verbosity lbi bi clbi =
               (mkVersion [9, 4])
               (compiler lbi)
               (maybeToFlag $ programPath <$> lookupProgram gccProgram (withPrograms lbi))
+        , -- The assumption that the C++ compiler is part of the toolchain is only since ghc-9.4.
+          ghcOptGppProgram =
+            ghcOptionsSince
+              (mkVersion [9, 4])
+              (compiler lbi)
+              (maybeToFlag $ programPath <$> lookupProgram gppProgram (withPrograms lbi))
         }
   where
     exe_paths =

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

@@ -519,7 +519,9 @@ data GhcOptions = GhcOptions
   , ghcOptFfiIncludes :: NubListR FilePath
   -- ^ Extra header files to include for old-style FFI; the @ghc -#include@ flag.
   , ghcOptCcProgram :: Flag FilePath
-  -- ^ Program to use for the C and C++ compiler; the @ghc -pgmc@ flag.
+  -- ^ Program to use for the C compiler; the @ghc -pgmc@ flag.
+  , ghcOptGppProgram :: Flag FilePath
+  -- ^ Program to use for the C++ compiler; the @ghc -pgmcxx@ flag.
   , ----------------------------
     -- Language and extensions
 
@@ -875,6 +877,7 @@ renderGhcOptions comp _platform@(Platform _arch os) opts
            in [cxxflag ++ opt | opt <- ghcOptCxxOptions opts]
         , ["-opta" ++ opt | opt <- ghcOptAsmOptions opts]
         , concat [["-pgmc", cc] | cc <- flag ghcOptCcProgram]
+        , concat [["-pgmcxx", cxx] | cxx <- flag ghcOptGppProgram]
         , -----------------
           -- Linker stuff
 

cabal-testsuite/PackageTests/FFI/ForeignOptsPgmcxx/Main.hs

@@ -0,0 +1,16 @@
+{-# LANGUAGE ForeignFunctionInterface #-}
+
+module Main where
+
+import Foreign.C (CInt (..))
+
+foreign import ccall "pgmcxxlib.h meaning_of_life_pgmcxx"
+  meaning_of_life_pgmcxx :: IO CInt
+
+main :: IO ()
+main = do
+  secret <- meaning_of_life_pgmcxx
+  -- The value 67 comes from __TESTOPT_PGMCXX__ - see cxx-wrapper.sh.
+  if secret == 67
+    then putStrLn ("The secret is " ++ show secret)
+    else error ("Expected value 67, got " ++ show secret)

cabal-testsuite/PackageTests/FFI/ForeignOptsPgmcxx/README.md

@@ -0,0 +1,7 @@
+# ForeignOptsPgmcxx
+
+This test case asserts that cabal passes the `-pgmcxx` GHC option to override the C++ compiler program.
+
+The cabal file sets `ghc-options: -pgmcxx scripts/cxx-wrapper.sh`, pointing GHC at a shell script wrapper (`scripts/cxx-wrapper.sh`) instead of the system C++ compiler. The wrapper adds `-D__TESTOPT_PGMCXX__=67` to every compilation and then delegates to the real `g++`. The C++ source requires `__TESTOPT_PGMCXX__` to be defined; if the wrapper is not used as the C++ compiler, the build fails with a `#error`.
+
+This test is skipped on Windows (no POSIX shell).

cabal-testsuite/PackageTests/FFI/ForeignOptsPgmcxx/cabal.out

@@ -0,0 +1 @@
+packages: .

cabal-testsuite/PackageTests/FFI/ForeignOptsPgmcxx/cabal.project

@@ -0,0 +1,9 @@
+import Test.Cabal.Prelude
+
+main = do
+  skipIfWindows "requires a POSIX shell script as the C++ compiler wrapper"
+  -- The assumption that the C++ compiler is part of the toolchain is only since ghc-9.4.
+  cabalTest $ recordMode DoNotRecord $ do
+    skipUnlessGhcVersion ">= 9.4"
+    cabal "v2-build" ["foreign-opts-pgmcxx-exe"]
+    withPlan $ runPlanExe "foreign-opts-pgmcxx" "foreign-opts-pgmcxx-exe" []

cabal-testsuite/PackageTests/FFI/ForeignOptsPgmcxx/cabal.test.hs

@@ -0,0 +1,9 @@
+#include "pgmcxxlib.h"
+
+#ifndef __TESTOPT_PGMCXX__
+#error "Did not get required __TESTOPT_PGMCXX__ from the -pgmcxx wrapper"
+#endif
+
+int meaning_of_life_pgmcxx() {
+    return __TESTOPT_PGMCXX__;
+}

cabal-testsuite/PackageTests/FFI/ForeignOptsPgmcxx/cxxbits/pgmcxxlib.cpp

@@ -0,0 +1,8 @@
+#ifndef PGMCXXLIB_H
+#define PGMCXXLIB_H
+extern "C" {
+
+int meaning_of_life_pgmcxx();
+
+}
+#endif

cabal-testsuite/PackageTests/FFI/ForeignOptsPgmcxx/cxxbits/pgmcxxlib.h

@@ -0,0 +1,15 @@
+cabal-version:       2.2
+name:                foreign-opts-pgmcxx
+version:             0.1
+build-type:          Simple
+
+executable foreign-opts-pgmcxx-exe
+  main-is:             Main.hs
+  build-depends:       base
+  default-language:    Haskell2010
+  include-dirs:        cxxbits
+  -- The stdc++ extra library option is not needed given that we now pass -pgmcxx by default.
+  -- Only gcc need stdc++ for C++ code, g++ doesn't need it.
+  -- extra-libraries:     stdc++
+  cxx-sources:         cxxbits/pgmcxxlib.cpp
+  ghc-options:         -pgmcxx scripts/cxx-wrapper.sh