CppTools: Allow interpreting ambiguous headers as C headers

...instead of C++ headers.

For the Clang Code Model this results in using "-x c-header" instead of
"-x c++-header".

This introduces a new option in Options > C++ > "Code Model" to
configure this.

Change-Id: I8a0ce8fa6155f5ef58743ebc7f1d0b500fbf6599
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

18 files changed, 172 insertions, 70 deletions

diff --git a/src/plugins/cpptools/baseeditordocumentparser.cpp b/src/plugins/cpptools/baseeditordocumentparser.cpp
index f346e05415..ca4d16e564 100644
--- a/src/plugins/cpptools/baseeditordocumentparser.cpp
+++ b/src/plugins/cpptools/baseeditordocumentparser.cpp
@@ -122,6 +122,7 @@ ProjectPart::Ptr BaseEditorDocumentParser::determineProjectPart(
         const Configuration &config,
         const State &state,
         const ProjectExplorer::Project *activeProject,
+        Language languagePreference,
         bool hasActiveProjectChanged)
 {
     Internal::ProjectPartChooser chooser;
@@ -141,6 +142,7 @@ ProjectPart::Ptr BaseEditorDocumentParser::determineProjectPart(
                           config.manuallySetProjectPart,
                           config.stickToPreviousProjectPart,
                           activeProject,
+                          languagePreference,
                           hasActiveProjectChanged);
 }
 
diff --git a/src/plugins/cpptools/baseeditordocumentparser.h b/src/plugins/cpptools/baseeditordocumentparser.h
index 11f76fa67d..92f919e96d 100644
--- a/src/plugins/cpptools/baseeditordocumentparser.h
+++ b/src/plugins/cpptools/baseeditordocumentparser.h
@@ -25,6 +25,7 @@
 
 #pragma once
 
+#include "cpplanguage.h"
 #include "cpptools_global.h"
 #include "cppworkingcopy.h"
 #include "projectpart.h"
@@ -55,15 +56,18 @@ public:
     struct UpdateParams {
         UpdateParams(const WorkingCopy &workingCopy,
                      const ProjectExplorer::Project *activeProject,
+                     Language languagePreference,
                      bool hasActiveProjectChanged)
             : workingCopy(workingCopy)
             , activeProject(activeProject)
+            , languagePreference(languagePreference)
             , hasActiveProjectChanged(hasActiveProjectChanged)
         {
         }
 
         WorkingCopy workingCopy;
         const ProjectExplorer::Project *activeProject = nullptr;
+        Language languagePreference = Language::Cxx;
         bool hasActiveProjectChanged = false;
     };
 
@@ -92,6 +96,7 @@ protected:
                                                  const Configuration &config,
                                                  const State &state,
                                                  const ProjectExplorer::Project *activeProject,
+                                                 Language languagePreference,
                                                  bool hasActiveProjectChanged);
 
     mutable QMutex m_stateAndConfigurationMutex;
diff --git a/src/plugins/cpptools/baseeditordocumentprocessor.cpp b/src/plugins/cpptools/baseeditordocumentprocessor.cpp
index bf68bc1c4e..b656c5798a 100644
--- a/src/plugins/cpptools/baseeditordocumentprocessor.cpp
+++ b/src/plugins/cpptools/baseeditordocumentprocessor.cpp
@@ -25,8 +25,11 @@
 
 #include "baseeditordocumentprocessor.h"
 
+#include "cppcodemodelsettings.h"
+#include "cpplanguage.h"
 #include "cppmodelmanager.h"
 #include "cpptoolsbridge.h"
+#include "cpptoolsreuse.h"
 #include "editordocumenthandle.h"
 
 #include <projectexplorer/session.h>
@@ -55,8 +58,13 @@ BaseEditorDocumentProcessor::~BaseEditorDocumentProcessor()
 
 void BaseEditorDocumentProcessor::run(bool hasActiveProjectChanged)
 {
+    const Language languagePreference = codeModelSettings()->interpretAmbigiousHeadersAsCHeaders()
+            ? Language::C
+            : Language::Cxx;
+
     runImpl({CppModelManager::instance()->workingCopy(),
              ProjectExplorer::SessionManager::startupProject(),
+             languagePreference,
              hasActiveProjectChanged});
 }
 
diff --git a/src/plugins/cpptools/builtineditordocumentparser.cpp b/src/plugins/cpptools/builtineditordocumentparser.cpp
index 77700e2cdf..5243e2a76b 100644
--- a/src/plugins/cpptools/builtineditordocumentparser.cpp
+++ b/src/plugins/cpptools/builtineditordocumentparser.cpp
@@ -81,6 +81,7 @@ void BuiltinEditorDocumentParser::updateImpl(const QFutureInterface<void> &futur
                                                  baseConfig,
                                                  baseState,
                                                  updateParams.activeProject,
+                                                 updateParams.languagePreference,
                                                  updateParams.hasActiveProjectChanged);
 
     if (state.forceSnapshotInvalidation) {
diff --git a/src/plugins/cpptools/builtinindexingsupport.cpp b/src/plugins/cpptools/builtinindexingsupport.cpp
index c4360f5fd7..6d3eac843b 100644
--- a/src/plugins/cpptools/builtinindexingsupport.cpp
+++ b/src/plugins/cpptools/builtinindexingsupport.cpp
@@ -158,7 +158,8 @@ void indexFindErrors(QFutureInterface<void> &future, const ParseParams params)
         // Parse the file as precisely as possible
         BuiltinEditorDocumentParser parser(file);
         parser.setReleaseSourceAndAST(false);
-        parser.update({CppModelManager::instance()->workingCopy(), nullptr, false});
+        parser.update({CppModelManager::instance()->workingCopy(), nullptr,
+                       Language::Cxx, false});
         CPlusPlus::Document::Ptr document = parser.document();
         QTC_ASSERT(document, return);
 
diff --git a/src/plugins/cpptools/cppcodemodelsettings.cpp b/src/plugins/cpptools/cppcodemodelsettings.cpp
index 43d8e5f812..a129fa61da 100644
--- a/src/plugins/cpptools/cppcodemodelsettings.cpp
+++ b/src/plugins/cpptools/cppcodemodelsettings.cpp
@@ -58,6 +58,9 @@ static QString clangDiagnosticConfigsArrayOptionsKey()
 static QString pchUsageKey()
 { return QLatin1String(Constants::CPPTOOLS_MODEL_MANAGER_PCH_USAGE); }
 
+static QString interpretAmbiguousHeadersAsCHeadersKey()
+{ return QLatin1String(Constants::CPPTOOLS_INTERPRET_AMBIGIUOUS_HEADERS_AS_C_HEADERS); }
+
 static QString skipIndexingBigFilesKey()
 { return QLatin1String(Constants::CPPTOOLS_SKIP_INDEXING_BIG_FILES); }
 
@@ -88,6 +91,10 @@ void CppCodeModelSettings::fromSettings(QSettings *s)
     const QVariant pchUsageVariant = s->value(pchUsageKey(), initialPchUsage());
     setPCHUsage(static_cast<PCHUsage>(pchUsageVariant.toInt()));
 
+    const QVariant interpretAmbiguousHeadersAsCHeaders
+            = s->value(interpretAmbiguousHeadersAsCHeadersKey(), false);
+    setInterpretAmbigiousHeadersAsCHeaders(interpretAmbiguousHeadersAsCHeaders.toBool());
+
     const QVariant skipIndexingBigFiles = s->value(skipIndexingBigFilesKey(), true);
     setSkipIndexingBigFiles(skipIndexingBigFiles.toBool());
 
@@ -116,6 +123,8 @@ void CppCodeModelSettings::toSettings(QSettings *s)
 
     s->setValue(clangDiagnosticConfigKey(), clangDiagnosticConfigId().toSetting());
     s->setValue(pchUsageKey(), pchUsage());
+
+    s->setValue(interpretAmbiguousHeadersAsCHeadersKey(), interpretAmbigiousHeadersAsCHeaders());
     s->setValue(skipIndexingBigFilesKey(), skipIndexingBigFiles());
     s->setValue(indexerFileSizeLimitKey(), indexerFileSizeLimitInMb());
 
@@ -166,6 +175,16 @@ void CppCodeModelSettings::emitChanged()
     emit changed();
 }
 
+bool CppCodeModelSettings::interpretAmbigiousHeadersAsCHeaders() const
+{
+    return m_interpretAmbigiousHeadersAsCHeaders;
+}
+
+void CppCodeModelSettings::setInterpretAmbigiousHeadersAsCHeaders(bool yesno)
+{
+    m_interpretAmbigiousHeadersAsCHeaders = yesno;
+}
+
 bool CppCodeModelSettings::skipIndexingBigFiles() const
 {
     return m_skipIndexingBigFiles;
diff --git a/src/plugins/cpptools/cppcodemodelsettings.h b/src/plugins/cpptools/cppcodemodelsettings.h
index c57ee8bbc4..4c6404e506 100644
--- a/src/plugins/cpptools/cppcodemodelsettings.h
+++ b/src/plugins/cpptools/cppcodemodelsettings.h
@@ -63,6 +63,9 @@ public:
     PCHUsage pchUsage() const;
     void setPCHUsage(PCHUsage pchUsage);
 
+    bool interpretAmbigiousHeadersAsCHeaders() const;
+    void setInterpretAmbigiousHeadersAsCHeaders(bool yesno);
+
     bool skipIndexingBigFiles() const;
     void setSkipIndexingBigFiles(bool yesno);
 
@@ -78,6 +81,7 @@ signals:
 
 private:
     PCHUsage m_pchUsage = PchUse_None;
+    bool m_interpretAmbigiousHeadersAsCHeaders = false;
     bool m_skipIndexingBigFiles = true;
     int m_indexerFileSizeLimitInMB = 5;
     ClangDiagnosticConfigs m_clangCustomDiagnosticConfigs;
diff --git a/src/plugins/cpptools/cppcodemodelsettingspage.cpp b/src/plugins/cpptools/cppcodemodelsettingspage.cpp
index e1792ed0b7..e6af115aed 100644
--- a/src/plugins/cpptools/cppcodemodelsettingspage.cpp
+++ b/src/plugins/cpptools/cppcodemodelsettingspage.cpp
@@ -56,18 +56,16 @@ void CppCodeModelSettingsWidget::setSettings(const QSharedPointer<CppCodeModelSe
 {
     m_settings = s;
 
+    setupGeneralWidgets();
     setupClangCodeModelWidgets();
-    setupPchCheckBox();
-    setupSkipIndexingFilesWidgets();
 }
 
 void CppCodeModelSettingsWidget::applyToSettings() const
 {
     bool changed = false;
 
+    changed |= applyGeneralWidgetsToSettings();
     changed |= applyClangCodeModelWidgetsToSettings();
-    changed |= applyPchCheckBoxToSettings();
-    changed |= applySkipIndexingFilesWidgets();
 
     if (changed)
         m_settings->toSettings(Core::ICore::settings());
@@ -88,16 +86,16 @@ void CppCodeModelSettingsWidget::setupClangCodeModelWidgets()
     m_ui->clangSettingsGroupBox->layout()->addWidget(m_clangDiagnosticConfigsWidget);
 }
 
-void CppCodeModelSettingsWidget::setupPchCheckBox() const
+void CppCodeModelSettingsWidget::setupGeneralWidgets()
 {
-    const bool ignorePch = m_settings->pchUsage() == CppCodeModelSettings::PchUse_None;
-    m_ui->ignorePCHCheckBox->setChecked(ignorePch);
-}
+    m_ui->interpretAmbiguousHeadersAsCHeaders->setChecked(
+                m_settings->interpretAmbigiousHeadersAsCHeaders());
 
-void CppCodeModelSettingsWidget::setupSkipIndexingFilesWidgets()
-{
     m_ui->skipIndexingBigFilesCheckBox->setChecked(m_settings->skipIndexingBigFiles());
     m_ui->bigFilesLimitSpinBox->setValue(m_settings->indexerFileSizeLimitInMb());
+
+    const bool ignorePch = m_settings->pchUsage() == CppCodeModelSettings::PchUse_None;
+    m_ui->ignorePCHCheckBox->setChecked(ignorePch);
 }
 
 bool CppCodeModelSettingsWidget::applyClangCodeModelWidgetsToSettings() const
@@ -122,39 +120,39 @@ bool CppCodeModelSettingsWidget::applyClangCodeModelWidgetsToSettings() const
     return settingsChanged;
 }
 
-bool CppCodeModelSettingsWidget::applyPchCheckBoxToSettings() const
+bool CppCodeModelSettingsWidget::applyGeneralWidgetsToSettings() const
 {
-    const bool newIgnorePch = m_ui->ignorePCHCheckBox->isChecked();
-    const bool previousIgnorePch = m_settings->pchUsage() == CppCodeModelSettings::PchUse_None;
-
-    if (newIgnorePch != previousIgnorePch) {
-        const CppCodeModelSettings::PCHUsage pchUsage = m_ui->ignorePCHCheckBox->isChecked()
-                ? CppCodeModelSettings::PchUse_None
-                : CppCodeModelSettings::PchUse_BuildSystem;
-        m_settings->setPCHUsage(pchUsage);
+    bool settingsChanged = false;
 
-        return true;
+    const bool newInterpretAmbiguousHeaderAsCHeaders
+            = m_ui->interpretAmbiguousHeadersAsCHeaders->isChecked();
+    if (m_settings->interpretAmbigiousHeadersAsCHeaders()
+            != newInterpretAmbiguousHeaderAsCHeaders) {
+        m_settings->setInterpretAmbigiousHeadersAsCHeaders(newInterpretAmbiguousHeaderAsCHeaders);
+        settingsChanged = true;
     }
 
-    return false;
-}
-
-bool CppCodeModelSettingsWidget::applySkipIndexingFilesWidgets() const
-{
-    bool settingsChanged = false;
-
     const bool newSkipIndexingBigFiles = m_ui->skipIndexingBigFilesCheckBox->isChecked();
     if (m_settings->skipIndexingBigFiles() != newSkipIndexingBigFiles) {
         m_settings->setSkipIndexingBigFiles(newSkipIndexingBigFiles);
         settingsChanged = true;
     }
-
     const int newFileSizeLimit = m_ui->bigFilesLimitSpinBox->value();
     if (m_settings->indexerFileSizeLimitInMb() != newFileSizeLimit) {
         m_settings->setIndexerFileSizeLimitInMb(newFileSizeLimit);
         settingsChanged = true;
     }
 
+    const bool newIgnorePch = m_ui->ignorePCHCheckBox->isChecked();
+    const bool previousIgnorePch = m_settings->pchUsage() == CppCodeModelSettings::PchUse_None;
+    if (newIgnorePch != previousIgnorePch) {
+        const CppCodeModelSettings::PCHUsage pchUsage = m_ui->ignorePCHCheckBox->isChecked()
+                ? CppCodeModelSettings::PchUse_None
+                : CppCodeModelSettings::PchUse_BuildSystem;
+        m_settings->setPCHUsage(pchUsage);
+        settingsChanged = true;
+    }
+
     return settingsChanged;
 }
 
diff --git a/src/plugins/cpptools/cppcodemodelsettingspage.h b/src/plugins/cpptools/cppcodemodelsettingspage.h
index fc249d4285..21911763a1 100644
--- a/src/plugins/cpptools/cppcodemodelsettingspage.h
+++ b/src/plugins/cpptools/cppcodemodelsettingspage.h
@@ -55,13 +55,11 @@ public:
     void applyToSettings() const;
 
 private:
+    void setupGeneralWidgets();
     void setupClangCodeModelWidgets();
-    void setupPchCheckBox() const;
-    void setupSkipIndexingFilesWidgets();
 
+    bool applyGeneralWidgetsToSettings() const;
     bool applyClangCodeModelWidgetsToSettings() const;
-    bool applyPchCheckBoxToSettings() const;
-    bool applySkipIndexingFilesWidgets() const;
 
 private:
     Ui::CppCodeModelSettingsPage *m_ui;
diff --git a/src/plugins/cpptools/cppcodemodelsettingspage.ui b/src/plugins/cpptools/cppcodemodelsettingspage.ui
index 34a946854b..025b91dba9 100644
--- a/src/plugins/cpptools/cppcodemodelsettingspage.ui
+++ b/src/plugins/cpptools/cppcodemodelsettingspage.ui
@@ -15,37 +15,19 @@
   </property>
   <layout class="QVBoxLayout" name="verticalLayout_2">
    <item>
-    <widget class="QLabel" name="clangCodeModelIsEnabledHint">
-     <property name="text">
-      <string>&lt;i&gt;The Clang Code Model is enabled because the corresponding plugin is loaded.&lt;/i&gt;</string>
-     </property>
-    </widget>
-   </item>
-   <item>
-    <widget class="QLabel" name="clangCodeModelIsDisabledHint">
-     <property name="text">
-      <string>&lt;i&gt;The Clang Code Model is disabled because the corresponding plugin is not loaded.&lt;/i&gt;</string>
-     </property>
-    </widget>
-   </item>
-   <item>
-    <widget class="QGroupBox" name="clangSettingsGroupBox">
-     <property name="title">
-      <string>Clang Code Model Warnings</string>
-     </property>
-     <property name="checkable">
-      <bool>false</bool>
-     </property>
-     <layout class="QVBoxLayout" name="verticalLayout_3"/>
-    </widget>
-   </item>
-   <item>
     <widget class="QGroupBox" name="anotherGroupBox">
      <property name="title">
-      <string>Files to Skip</string>
+      <string>General</string>
      </property>
      <layout class="QVBoxLayout" name="verticalLayout">
       <item>
+       <widget class="QCheckBox" name="interpretAmbiguousHeadersAsCHeaders">
+        <property name="text">
+         <string>Interpret ambiguous headers as C headers</string>
+        </property>
+       </widget>
+      </item>
+      <item>
        <widget class="QCheckBox" name="ignorePCHCheckBox">
         <property name="toolTip">
          <string>&lt;html&gt;&lt;head/&gt;&lt;body&gt;&lt;p&gt;When pre-compiled headers are not ignored, the parsing for code completion and semantic highlighting will process the pre-compiled header before processing any file.&lt;/p&gt;&lt;/body&gt;&lt;/html&gt;</string>
@@ -102,6 +84,31 @@
     </widget>
    </item>
    <item>
+    <widget class="QLabel" name="clangCodeModelIsEnabledHint">
+     <property name="text">
+      <string>&lt;i&gt;The Clang Code Model is enabled because the corresponding plugin is loaded.&lt;/i&gt;</string>
+     </property>
+    </widget>
+   </item>
+   <item>
+    <widget class="QLabel" name="clangCodeModelIsDisabledHint">
+     <property name="text">
+      <string>&lt;i&gt;The Clang Code Model is disabled because the corresponding plugin is not loaded.&lt;/i&gt;</string>
+     </property>
+    </widget>
+   </item>
+   <item>
+    <widget class="QGroupBox" name="clangSettingsGroupBox">
+     <property name="title">
+      <string>Clang Code Model Warnings</string>
+     </property>
+     <property name="checkable">
+      <bool>false</bool>
+     </property>
+     <layout class="QVBoxLayout" name="verticalLayout_3"/>
+    </widget>
+   </item>
+   <item>
     <spacer name="verticalSpacer">
      <property name="orientation">
       <enum>Qt::Vertical</enum>
diff --git a/src/plugins/cpptools/cppcompletionassist.cpp b/src/plugins/cpptools/cppcompletionassist.cpp
index 73800a094d..30b1a0d9dc 100644
--- a/src/plugins/cpptools/cppcompletionassist.cpp
+++ b/src/plugins/cpptools/cppcompletionassist.cpp
@@ -2122,7 +2122,10 @@ void CppCompletionAssistInterface::getCppSpecifics() const
     m_gotCppSpecifics = true;
 
     if (m_parser) {
-        m_parser->update({CppTools::CppModelManager::instance()->workingCopy(), nullptr, false});
+        m_parser->update({CppTools::CppModelManager::instance()->workingCopy(),
+                          nullptr,
+                          Language::Cxx,
+                          false});
         m_snapshot = m_parser->snapshot();
         m_headerPaths = m_parser->headerPaths();
     }
diff --git a/src/plugins/cpptools/cpplanguage.h b/src/plugins/cpptools/cpplanguage.h
new file mode 100644
index 0000000000..2a84641d4d
--- /dev/null
+++ b/src/plugins/cpptools/cpplanguage.h
@@ -0,0 +1,32 @@
+/****************************************************************************
+**
+** Copyright (C) 2016 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#pragma once
+
+namespace CppTools {
+
+enum class Language { C, Cxx };
+
+} // namespace CppTools
diff --git a/src/plugins/cpptools/cppmodelmanager_test.cpp b/src/plugins/cpptools/cppmodelmanager_test.cpp
index d5f0b2fc09..4db38959c3 100644
--- a/src/plugins/cpptools/cppmodelmanager_test.cpp
+++ b/src/plugins/cpptools/cppmodelmanager_test.cpp
@@ -877,7 +877,8 @@ void CppToolsPlugin::test_modelmanager_precompiled_headers()
         BaseEditorDocumentParser::Configuration config = parser->configuration();
         config.usePrecompiledHeaders = true;
         parser->setConfiguration(config);
-        parser->update({CppModelManager::instance()->workingCopy(), nullptr, false});
+        parser->update({CppModelManager::instance()->workingCopy(), nullptr,
+                        Language::Cxx, false});
 
         // Check if defines from pch are considered
         Document::Ptr document = mm->document(fileName);
@@ -955,7 +956,8 @@ void CppToolsPlugin::test_modelmanager_defines_per_editor()
         BaseEditorDocumentParser::Configuration config = parser->configuration();
         config.editorDefines = editorDefines.toUtf8();
         parser->setConfiguration(config);
-        parser->update({CppModelManager::instance()->workingCopy(), nullptr, false});
+        parser->update({CppModelManager::instance()->workingCopy(), nullptr,
+                        Language::Cxx, false});
 
         Document::Ptr doc = mm->document(main1File);
         QCOMPARE(nameOfFirstDeclaration(doc), firstDeclarationName);
diff --git a/src/plugins/cpptools/cppprojectpartchooser.cpp b/src/plugins/cpptools/cppprojectpartchooser.cpp
index 19c40c5fe0..5f7d3de137 100644
--- a/src/plugins/cpptools/cppprojectpartchooser.cpp
+++ b/src/plugins/cpptools/cppprojectpartchooser.cpp
@@ -31,25 +31,39 @@
 namespace CppTools {
 namespace Internal {
 
-static int priority(const ProjectPart &projectPart, const ProjectExplorer::Project *activeProject)
+static bool isPreferredLanguage(const ProjectPart &projectPart, Language preference)
+{
+    const bool isCProjectPart = projectPart.languageVersion <= ProjectPart::LatestCVersion;
+    return (preference == Language::C && isCProjectPart)
+        || (preference == Language::Cxx && !isCProjectPart);
+}
+
+static int priority(const ProjectPart &projectPart,
+                    const ProjectExplorer::Project *activeProject,
+                    Language languagePreference)
 {
     int thePriority = 0;
 
     if (projectPart.project == activeProject)
-        thePriority += 10;
+        thePriority += 100;
 
     if (projectPart.selectedForBuilding)
+        thePriority += 10;
+
+    if (isPreferredLanguage(projectPart, languagePreference))
         thePriority += 1;
 
     return thePriority;
 }
 
 static ProjectPart::Ptr chooseFromMultiple(const QList<ProjectPart::Ptr> &projectParts,
-                                           const ProjectExplorer::Project *activeProject)
+                                           const ProjectExplorer::Project *activeProject,
+                                           Language languagePreference)
 {
     QList<ProjectPart::Ptr> projectPartsPrioritized = projectParts;
-    const auto lessThan = [activeProject] (const ProjectPart::Ptr &p1, const ProjectPart::Ptr &p2) {
-        return priority(*p1, activeProject) > priority(*p2, activeProject);
+    const auto lessThan = [&] (const ProjectPart::Ptr &p1, const ProjectPart::Ptr &p2) {
+        return priority(*p1, activeProject, languagePreference)
+             > priority(*p2, activeProject, languagePreference);
     };
     std::stable_sort(projectPartsPrioritized.begin(), projectPartsPrioritized.end(), lessThan);
 
@@ -61,6 +75,7 @@ ProjectPart::Ptr ProjectPartChooser::choose(const QString &filePath,
                                             const ProjectPart::Ptr &manuallySetProjectPart,
                                             bool stickToPreviousProjectPart,
                                             const ProjectExplorer::Project *activeProject,
+                                            Language languagePreference,
                                             bool projectHasChanged) const
 {
     QTC_CHECK(m_projectPartsForFile);
@@ -85,10 +100,10 @@ ProjectPart::Ptr ProjectPartChooser::choose(const QString &filePath,
             // Fall-back step 2: Use fall-back part from the model manager:
             projectPart = m_fallbackProjectPart();
         else
-            projectPart = chooseFromMultiple(projectParts, activeProject);
+            projectPart = chooseFromMultiple(projectParts, activeProject, languagePreference);
     } else {
         if (projectHasChanged || !projectParts.contains(projectPart))
-            projectPart = chooseFromMultiple(projectParts, activeProject);
+            projectPart = chooseFromMultiple(projectParts, activeProject, languagePreference);
     }
 
     return projectPart;
diff --git a/src/plugins/cpptools/cppprojectpartchooser.h b/src/plugins/cpptools/cppprojectpartchooser.h
index 1ad6da10af..e0a8581c34 100644
--- a/src/plugins/cpptools/cppprojectpartchooser.h
+++ b/src/plugins/cpptools/cppprojectpartchooser.h
@@ -25,6 +25,7 @@
 
 #pragma once
 
+#include "cpplanguage.h"
 #include "projectpart.h"
 
 #include <functional>
@@ -52,6 +53,7 @@ public:
                             const ProjectPart::Ptr &manuallySetProjectPart,
                             bool stickToPreviousProjectPart,
                             const ProjectExplorer::Project *activeProject,
+                            Language languagePreference,
                             bool projectHasChanged) const;
 
 private:
diff --git a/src/plugins/cpptools/cpptools.pro b/src/plugins/cpptools/cpptools.pro
index 00611dbca1..9b3b064b77 100644
--- a/src/plugins/cpptools/cpptools.pro
+++ b/src/plugins/cpptools/cpptools.pro
@@ -34,6 +34,7 @@ HEADERS += \
     cppfunctionsfilter.h \
     cppincludesfilter.h \
     cppindexingsupport.h \
+    cpplanguage.h \
     cpplocalsymbols.h \
     cpplocatordata.h \
     cpplocatorfilter.h \
diff --git a/src/plugins/cpptools/cpptools.qbs b/src/plugins/cpptools/cpptools.qbs
index 53ff39b5e9..f7047ed28d 100644
--- a/src/plugins/cpptools/cpptools.qbs
+++ b/src/plugins/cpptools/cpptools.qbs
@@ -61,6 +61,7 @@ Project {
             "cppfunctionsfilter.cpp", "cppfunctionsfilter.h",
             "cppincludesfilter.cpp", "cppincludesfilter.h",
             "cppindexingsupport.cpp", "cppindexingsupport.h",
+            "cpplanguage.h",
             "cpplocalsymbols.cpp", "cpplocalsymbols.h",
             "cpplocatordata.cpp", "cpplocatordata.h",
             "cpplocatorfilter.cpp", "cpplocatorfilter.h",
diff --git a/src/plugins/cpptools/cpptoolsconstants.h b/src/plugins/cpptools/cpptoolsconstants.h
index 11a5f1a1ab..18192317ce 100644
--- a/src/plugins/cpptools/cpptoolsconstants.h
+++ b/src/plugins/cpptools/cpptoolsconstants.h
@@ -51,10 +51,13 @@ enum { lowerCaseFilesDefault = 1 };
 const char CPPTOOLS_SORT_EDITOR_DOCUMENT_OUTLINE[] = "SortedMethodOverview";
 const char CPPTOOLS_SHOW_INFO_BAR_FOR_HEADER_ERRORS[] = "ShowInfoBarForHeaderErrors";
 const char CPPTOOLS_MODEL_MANAGER_PCH_USAGE[] = "PCHUsage";
+const char CPPTOOLS_INTERPRET_AMBIGIUOUS_HEADERS_AS_C_HEADERS[]
+    = "InterpretAmbiguousHeadersAsCHeaders";
 const char CPPTOOLS_SKIP_INDEXING_BIG_FILES[] = "SkipIndexingBigFiles";
 const char CPPTOOLS_INDEXER_FILE_SIZE_LIMIT[] = "IndexerFileSizeLimit";
 
-const char CPP_CLANG_BUILTIN_CONFIG_ID_EVERYTHING_WITH_EXCEPTIONS[] = "Builtin.EverythingWithExceptions";
+const char CPP_CLANG_BUILTIN_CONFIG_ID_EVERYTHING_WITH_EXCEPTIONS[]
+    = "Builtin.EverythingWithExceptions";
 
 const char CPP_CODE_STYLE_SETTINGS_ID[] = "A.Cpp.Code Style";
 const char CPP_CODE_STYLE_SETTINGS_NAME[] = QT_TRANSLATE_NOOP("CppTools", "Code Style");