system-cxx-std-lib: use cxx instead of c

compiler/GHC/Driver/Config/Linker.hs

@@ -20,8 +20,8 @@ initFrameworkOpts dflags = FrameworkOpts
   }
 
 -- | Initialize linker configuration from DynFlags
-initLinkerConfig :: DynFlags -> LinkerConfig
-initLinkerConfig dflags =
+initLinkerConfig :: DynFlags -> Bool -> LinkerConfig
+initLinkerConfig dflags require_cxx =
   let
     -- see Note [Solaris linker]
     ld_filter = case platformOS (targetPlatform dflags) of
@@ -46,8 +46,13 @@ initLinkerConfig dflags =
     (p,pre_args) = pgm_l dflags
     post_args    = map Option (getOpts dflags opt_l)
 
+    -- sneakily switch to C++ compiler when we need C++ standard lib
+    -- FIXME: ld flags may be totally inappropriate for the C++ compiler?
+    ld_prog      = if require_cxx then pgm_cxx dflags else p
+
+
   in LinkerConfig
-    { linkerProgram     = p
+    { linkerProgram     = ld_prog
     , linkerOptionsPre  = pre_args
     , linkerOptionsPost = post_args
     , linkerTempDir     = tmpDir dflags

compiler/GHC/Driver/Config/StgToJS.hs

@@ -34,7 +34,7 @@ initStgToJSConfig dflags = StgToJSConfig
   , csRuntimeAssert   = False
   -- settings
   , csContext         = initSDocContext dflags defaultDumpStyle
-  , csLinkerConfig    = initLinkerConfig dflags
+  , csLinkerConfig    = initLinkerConfig dflags False -- no C++ linking
   }
 
 -- | Default linker configuration

compiler/GHC/Linker/Dynamic.hs

@@ -24,6 +24,7 @@ import GHC.Linker.Unit
 import GHC.Linker.External
 import GHC.Utils.Logger
 import GHC.Utils.TmpFs
+import GHC.Data.FastString
 
 import Control.Monad (when)
 import System.FilePath
@@ -105,7 +106,8 @@ linkDynLib logger tmpfs dflags0 unit_env o_files dep_packages
     pkg_framework_opts <- getUnitFrameworkOpts unit_env (map unitId pkgs)
     let framework_opts = getFrameworkOpts (initFrameworkOpts dflags) platform
 
-    let linker_config = initLinkerConfig dflags
+    let require_cxx = any ((==) (PackageName (fsLit "system-cxx-std-lib")) . unitPackageName) pkgs
+    let linker_config = initLinkerConfig dflags require_cxx
 
     case os of
         OSMinGW32 -> do

compiler/GHC/Linker/Static.hs

@@ -33,6 +33,8 @@ import GHC.Linker.Static.Utils
 import GHC.Driver.Config.Linker
 import GHC.Driver.Session
 
+import GHC.Data.FastString
+
 import System.FilePath
 import System.Directory
 import Control.Monad
@@ -192,7 +194,9 @@ linkBinary' staticLink logger tmpfs dflags unit_env o_files dep_units = do
       OSMinGW32 | gopt Opt_GenManifest dflags -> maybeCreateManifest logger tmpfs dflags output_fn
       _                                       -> return []
 
-    let linker_config = initLinkerConfig dflags
+    let require_cxx = any ((==) (PackageName (fsLit "system-cxx-std-lib")) . unitPackageName) pkgs
+
+    let linker_config = initLinkerConfig dflags require_cxx
     let link dflags args = do
           runLink logger tmpfs linker_config args
           -- Make sure to honour -fno-use-rpaths if set on darwin as well; see #20004

libraries/system-cxx-std-lib/system-cxx-std-lib.cabal

@@ -0,0 +1,21 @@
+cabal-version: 2.0
+name: system-cxx-std-lib
+version: 1.0
+license: BSD-3-Clause
+synopsis:             A placeholder for the system's C++ standard library implementation.
+description:          Building against C++ libraries requires that the C++ standard
+                      library be included when linking. Typically when compiling a C++
+                      project this is done automatically by the C++ compiler. However,
+                      as GHC uses the C compiler for linking, users needing the C++
+                      standard library must declare this dependency explicitly.
+                      .
+                      This "virtual" package can be used to depend upon the host system's
+                      C++ standard library implementation in a platform agnostic manner.
+category:             System
+build-type: Simple
+
+library
+  -- empty library: this is just a placeholder for GHC to use to inject C++
+  -- standard libraries when linking with the C toolchain, or to directly use
+  -- the C++ toolchain to link.
+