[Clang] Take libstdc++ into account during GCC detection

clang/docs/ReleaseNotes.rst

@@ -37,6 +37,22 @@ latest release, please see the `Clang Web Site <https://clang.llvm.org>`_ or the
 Potentially Breaking Changes
 ============================
 
+- Clang will now emit a warning if the auto-detected GCC installation
+  directory (i.e. the one with the largest version number) does not
+  contain libstdc++ include directories although a "complete" GCC
+  installation directory containing the include directories is
+  available. It is planned to change the auto-detection to prefer the
+  "complete" directory in the future.  The warning will disappear if
+  the libstdc++ include directories are either installed or removed
+  for all GCC installation directories considered by the
+  auto-detection; see the output of ``clang -v`` for a list of those
+  directories. If the GCC installations cannot be modified and
+  maintaining the current choice of the auto-detection is desired, the
+  GCC installation directory can be selected explicitly using the
+  ``--gcc-install-dir`` command line argument. This will silence the
+  warning. It can also be disabled using the
+  ``-Wno-gcc-install-dir-libstdcxx`` command line flag.
+
 C/C++ Language Potentially Breaking Changes
 -------------------------------------------
 

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -885,4 +885,9 @@ def warn_drv_openacc_without_cir
     : Warning<"OpenACC directives will result in no runtime behavior; use "
               "-fclangir to enable runtime effect">,
       InGroup<SourceUsesOpenACC>;
+
+def warn_drv_gcc_install_dir_libstdcxx : Warning<
+    "future releases of the clang compiler will prefer GCC installations "
+    "containing libstdc++ include directories; '%0' would be chosen over '%1'">,
+    InGroup<DiagGroup<"gcc-install-dir-libstdcxx">>;
 }

clang/include/clang/Driver/ToolChain.h

@@ -224,9 +224,6 @@ class ToolChain {
   static void addSystemFrameworkInclude(const llvm::opt::ArgList &DriverArgs,
                                         llvm::opt::ArgStringList &CC1Args,
                                         const Twine &Path);
-  static void addSystemInclude(const llvm::opt::ArgList &DriverArgs,
-                               llvm::opt::ArgStringList &CC1Args,
-                               const Twine &Path);
   static void addExternCSystemInclude(const llvm::opt::ArgList &DriverArgs,
                                       llvm::opt::ArgStringList &CC1Args,
                                       const Twine &Path);
@@ -246,6 +243,9 @@ class ToolChain {
   ///@}
 
 public:
+  static void addSystemInclude(const llvm::opt::ArgList &DriverArgs,
+                               llvm::opt::ArgStringList &CC1Args,
+                               const Twine &Path);
   virtual ~ToolChain();
 
   // Accessors

clang/lib/Driver/ToolChain.cpp

@@ -1409,13 +1409,6 @@ void ToolChain::addSystemFrameworkInclude(const llvm::opt::ArgList &DriverArgs,
   CC1Args.push_back(DriverArgs.MakeArgString(Path));
 }
 
-/// Utility function to add a system include directory to CC1 arguments.
-void ToolChain::addSystemInclude(const ArgList &DriverArgs,
-                                 ArgStringList &CC1Args, const Twine &Path) {
-  CC1Args.push_back("-internal-isystem");
-  CC1Args.push_back(DriverArgs.MakeArgString(Path));
-}
-
 /// Utility function to add a system include directory with extern "C"
 /// semantics to CC1 arguments.
 ///
@@ -1438,6 +1431,14 @@ void ToolChain::addExternCSystemIncludeIfExists(const ArgList &DriverArgs,
     addExternCSystemInclude(DriverArgs, CC1Args, Path);
 }
 
+/// Utility function to add a system include directory to CC1 arguments.
+/*static*/ void ToolChain::addSystemInclude(const ArgList &DriverArgs,
+                                            ArgStringList &CC1Args,
+                                            const Twine &Path) {
+  CC1Args.push_back("-internal-isystem");
+  CC1Args.push_back(DriverArgs.MakeArgString(Path));
+}
+
 /// Utility function to add a list of system framework directories to CC1.
 void ToolChain::addSystemFrameworkIncludes(const ArgList &DriverArgs,
                                            ArgStringList &CC1Args,