[Clang] Take libstdc++ into account during GCC detection (#145056)

The Generic_GCC::GCCInstallationDetector class picks the GCC
installation directory with the largest version number. Since the
location of the libstdc++ include directories is tied to the GCC
version, this can break C++ compilation if the libstdc++ headers for
this particular GCC version are not available. Linux distributions tend
to package the libstdc++ headers separately from GCC. This frequently
leads to situations in which a newer version of GCC gets installed as a
dependency of another package without installing the corresponding
libstdc++ package. Clang then fails to compile C++ code because it
cannot find the libstdc++ headers. Since libstdc++ headers are in fact
installed on the system, the GCC installation continues to work, the
user may not be aware of the details of the GCC detection, and the
compiler does not recognize the situation and emit a warning, this
behavior can be hard to understand - as witnessed by many related bug
reports over the years.

The goal of this work is to change the GCC detection to prefer GCC
installations that contain libstdc++ include directories over those
which do not. This should happen regardless of the input language since
picking different GCC installations for a build that mixes C and C++
might lead to incompatibilities.
Any change to the GCC installation detection will probably have a
negative impact on some users. For instance, for a C user who relies on
using the GCC installation with the largest version number, it might
become necessary to use the --gcc-install-dir option to ensure that this
GCC version is selected.
This seems like an acceptable trade-off given that the situation for
users who do not have any special demands on the particular GCC
installation directory would be improved significantly.
 
This patch does not yet change the automatic GCC installation directory
choice. Instead, it does introduce a warning that informs the user about
the future change if the chosen GCC installation directory differs from
the one that would be chosen if the libstdc++ headers are taken into
account.

See also this related Discourse discussion:
https://discourse.llvm.org/t/rfc-take-libstdc-into-account-during-gcc-detection/86992.

Author: frederik-h

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,