Clang: Avoid consuming gcc internal include paths

Given the (default) include paths of GCC, e.g.

 /usr/include/c++/7
 /usr/include/x86_64-linux-gnu/c++/7
 /usr/include/c++/7/backward
 /usr/lib/gcc/x86_64-linux-gnu/7/include
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/7/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include

discard gcc-internal paths like /usr/lib/gcc/x86_64-linux-gnu/7/include
as they are not relevant for clang and even confuse it with regard to
 #include_next.

Paths below the gcc install dir are considered as gcc-internal. The
install dir is queried with

  $ gcc -print-search-dirs

Some GCC distributions, like MinGW, ship the standard library headers in
the install dir. Ensure to not discard these.

Fixes: QTCREATORBUG-22898
Change-Id: Ia85258fb01b72ad073e71390e003fe8268e3b01f
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/cpptools/cppcodemodelinspectordumper.cpp

@@ -529,6 +529,7 @@ void Dumper::dumpProjectInfos( const QList<ProjectInfo> &projectInfos)
             m_out << i3 << "ToolChain Type         : " << part->toolchainType.toString() << "\n";
             m_out << i3 << "ToolChain Target Triple: " << part->toolChainTargetTriple << "\n";
             m_out << i3 << "ToolChain Word Width   : " << part->toolChainWordWidth << "\n";
+            m_out << i3 << "ToolChain Install Dir  : " << part->toolChainInstallDir << "\n";
             m_out << i3 << "Compiler Flags         : " << part->compilerFlags.join(", ") << "\n";
             m_out << i3 << "Selected For Building  : " << part->selectedForBuilding << "\n";
             m_out << i3 << "Build System Target    : " << part->buildSystemTarget << "\n";

src/plugins/cpptools/cppprojectinfogenerator.cpp

@@ -167,6 +167,7 @@ ProjectPart::Ptr ProjectInfoGenerator::createProjectPart(
     part->isMsvc2015Toolchain = tcInfo.isMsvc2015ToolChain;
     part->toolChainWordWidth = tcInfo.wordWidth == 64 ? ProjectPart::WordWidth64Bit
                                                       : ProjectPart::WordWidth32Bit;
+    part->toolChainInstallDir = tcInfo.installDir;
     part->toolChainTargetTriple = tcInfo.targetTriple;
     part->extraCodeModelFlags = tcInfo.extraCodeModelFlags;
     part->compilerFlags = flags.commandLineFlags;

src/plugins/cpptools/headerpathfilter.cpp

@@ -30,9 +30,12 @@
 #endif
 
 #include <projectexplorer/project.h>
+#include <projectexplorer/projectexplorerconstants.h>
 
 #include <QRegularExpression>
 
+#include <utils/algorithm.h>
+
 namespace CppTools {
 
 using ProjectExplorer::HeaderPath;
@@ -57,6 +60,31 @@ bool HeaderPathFilter::isProjectHeaderPath(const QString &path) const
     return path.startsWith(projectDirectory) || path.startsWith(buildDirectory);
 }
 
+void HeaderPathFilter::removeGccInternalIncludePaths()
+{
+    if (projectPart.toolchainType != ProjectExplorer::Constants::GCC_TOOLCHAIN_TYPEID
+        && projectPart.toolchainType != ProjectExplorer::Constants::MINGW_TOOLCHAIN_TYPEID) {
+        return;
+    }
+
+    if (projectPart.toolChainInstallDir.isEmpty())
+        return;
+
+    const Utils::FilePath gccInstallDir = projectPart.toolChainInstallDir;
+    auto isGccInternalInclude = [gccInstallDir](const HeaderPath &headerPath){
+        const auto includePath = Utils::FilePath::fromString(headerPath.path);
+        if (includePath.isChildOf(gccInstallDir)) {
+           const QString remainingPath = headerPath.path.mid(gccInstallDir.toString().size());
+           // MinGW ships the standard library headers in "<installdir>/include/c++".
+           // Ensure that we do not remove include paths pointing there.
+           return !remainingPath.startsWith("/include/c++");
+        }
+        return false;
+    };
+
+    Utils::erase(builtInHeaderPaths, isGccInternalInclude);
+}
+
 void HeaderPathFilter::filterHeaderPath(const ProjectExplorer::HeaderPath &headerPath)
 {
     if (headerPath.path.isEmpty())
@@ -135,6 +163,7 @@ void removeClangSystemHeaderPaths(HeaderPaths &headerPaths)
 void HeaderPathFilter::tweakHeaderPaths()
 {
     removeClangSystemHeaderPaths(builtInHeaderPaths);
+    removeGccInternalIncludePaths();
 
     auto split = resourceIterator(builtInHeaderPaths,
                                   projectPart.toolChainTargetTriple.contains("darwin"));

src/plugins/cpptools/headerpathfilter.h

@@ -57,6 +57,8 @@ private:
 
     bool isProjectHeaderPath(const QString &path) const;
 
+    void removeGccInternalIncludePaths();
+
     static QString ensurePathWithSlashEnding(const QString &path);
 
 public:

src/plugins/cpptools/projectpart.h

@@ -42,6 +42,7 @@
 #include <cplusplus/Token.h>
 
 #include <utils/cpplanguage_details.h>
+#include <utils/fileutils.h>
 
 #include <QString>
 #include <QSharedPointer>
@@ -108,6 +109,7 @@ public:
     bool isMsvc2015Toolchain = false;
     QString toolChainTargetTriple;
     ToolChainWordWidth toolChainWordWidth = WordWidth32Bit;
+    Utils::FilePath toolChainInstallDir;
     ProjectExplorer::WarningFlags warningFlags = ProjectExplorer::WarningFlags::Default;
 
     // Misc