Genex: Fix COMPILE_LANGUAGE in SYSTEM include directories

When evaluating `INTERFACE_SYSTEM_INCLUDE_DIRECTORIES`, or evaluating
`INTERFACE_INCLUDE_DIRECTORIES` on an imported target, thread the
compile language through to the generator expression evaluator so
that it can support `$<COMPILE_LANGUAGE:...>`.

Fixes: #17811

Source/cmGeneratorTarget.cxx

@@ -384,14 +384,15 @@ static void handleSystemIncludesDep(
   cmLocalGenerator* lg, cmGeneratorTarget const* depTgt,
   const std::string& config, cmGeneratorTarget const* headTarget,
   cmGeneratorExpressionDAGChecker* dagChecker,
-  std::vector<std::string>& result, bool excludeImported)
+  std::vector<std::string>& result, bool excludeImported,
+  std::string const& language)
 {
   if (const char* dirs =
         depTgt->GetProperty("INTERFACE_SYSTEM_INCLUDE_DIRECTORIES")) {
     cmGeneratorExpression ge;
     cmSystemTools::ExpandListArgument(
       ge.Parse(dirs)->Evaluate(lg, config, false, headTarget, depTgt,
-                               dagChecker),
+                               dagChecker, language),
       result);
   }
   if (!depTgt->IsImported() || excludeImported) {
@@ -403,7 +404,7 @@ static void handleSystemIncludesDep(
     cmGeneratorExpression ge;
     cmSystemTools::ExpandListArgument(
       ge.Parse(dirs)->Evaluate(lg, config, false, headTarget, depTgt,
-                               dagChecker),
+                               dagChecker, language),
       result);
   }
 }
@@ -735,7 +736,8 @@ const char* cmGeneratorTarget::GetLocationForBuild() const
 }
 
 bool cmGeneratorTarget::IsSystemIncludeDirectory(
-  const std::string& dir, const std::string& config) const
+  const std::string& dir, const std::string& config,
+  const std::string& language) const
 {
   assert(this->GetType() != cmStateEnums::INTERFACE_LIBRARY);
   std::string config_upper;
@@ -758,7 +760,7 @@ bool cmGeneratorTarget::IsSystemIncludeDirectory(
       cmGeneratorExpression ge;
       cmSystemTools::ExpandListArgument(
         ge.Parse(it)->Evaluate(this->LocalGenerator, config, false, this,
-                               &dagChecker),
+                               &dagChecker, language),
         result);
     }
 
@@ -766,7 +768,7 @@ bool cmGeneratorTarget::IsSystemIncludeDirectory(
       this->GetLinkImplementationClosure(config);
     for (cmGeneratorTarget const* dep : deps) {
       handleSystemIncludesDep(this->LocalGenerator, dep, config, this,
-                              &dagChecker, result, excludeImported);
+                              &dagChecker, result, excludeImported, language);
     }
 
     std::for_each(result.begin(), result.end(),

Source/cmGeneratorTarget.h

@@ -413,7 +413,8 @@ public:
                              const std::string& language) const;
 
   bool IsSystemIncludeDirectory(const std::string& dir,
-                                const std::string& config) const;
+                                const std::string& config,
+                                const std::string& language) const;
 
   /** Add the target output files to the global generator manifest.  */
   void ComputeTargetManifest(const std::string& config) const;

Source/cmGlobalXCodeGenerator.cxx

@@ -2035,7 +2035,8 @@ void cmGlobalXCodeGenerator::CreateBuildSettings(cmGeneratorTarget* gtgt,
       if (emitted.insert(frameworkDir).second) {
         std::string incpath = this->XCodeEscapePath(frameworkDir);
         if (emitSystemIncludes &&
-            gtgt->IsSystemIncludeDirectory(frameworkDir, configName)) {
+            gtgt->IsSystemIncludeDirectory(frameworkDir, configName,
+                                           langForPreprocessor)) {
           sysfdirs.Add(incpath);
         } else {
           fdirs.Add(incpath);
@@ -2044,7 +2045,8 @@ void cmGlobalXCodeGenerator::CreateBuildSettings(cmGeneratorTarget* gtgt,
     } else {
       std::string incpath = this->XCodeEscapePath(include);
       if (emitSystemIncludes &&
-          gtgt->IsSystemIncludeDirectory(include, configName)) {
+          gtgt->IsSystemIncludeDirectory(include, configName,
+                                         langForPreprocessor)) {
         sysdirs.Add(incpath);
       } else {
         dirs.Add(incpath);
@@ -2057,7 +2059,8 @@ void cmGlobalXCodeGenerator::CreateBuildSettings(cmGeneratorTarget* gtgt,
       if (emitted.insert(fwDir).second) {
         std::string incpath = this->XCodeEscapePath(fwDir);
         if (emitSystemIncludes &&
-            gtgt->IsSystemIncludeDirectory(fwDir, configName)) {
+            gtgt->IsSystemIncludeDirectory(fwDir, configName,
+                                           langForPreprocessor)) {
           sysfdirs.Add(incpath);
         } else {
           fdirs.Add(incpath);

Source/cmLocalGenerator.cxx

@@ -718,7 +718,7 @@ std::string cmLocalGenerator::GetIncludeFlags(
       frameworkDir = cmSystemTools::CollapseFullPath(frameworkDir);
       if (emitted.insert(frameworkDir).second) {
         if (sysFwSearchFlag && target &&
-            target->IsSystemIncludeDirectory(i, config)) {
+            target->IsSystemIncludeDirectory(i, config, lang)) {
           includeFlags << sysFwSearchFlag;
         } else {
           includeFlags << fwSearchFlag;
@@ -731,7 +731,7 @@ std::string cmLocalGenerator::GetIncludeFlags(
 
     if (!flagUsed || repeatFlag) {
       if (sysIncludeFlag && target &&
-          target->IsSystemIncludeDirectory(i, config)) {
+          target->IsSystemIncludeDirectory(i, config, lang)) {
         includeFlags << sysIncludeFlag;
       } else {
         includeFlags << includeFlag;

Source/cmServerProtocol.cxx

@@ -725,8 +725,9 @@ static Json::Value DumpSourceFilesList(
         lg->AppendIncludeDirectories(includes, evaluatedIncludes, *file);
 
         for (const auto& include : includes) {
-          fileData.IncludePathList.push_back(std::make_pair(
-            include, target->IsSystemIncludeDirectory(include, config)));
+          fileData.IncludePathList.push_back(
+            std::make_pair(include, target->IsSystemIncludeDirectory(
+                                      include, config, fileData.Language)));
         }
       }
 
@@ -1005,7 +1006,7 @@ static Json::Value DumpTarget(cmGeneratorTarget* target,
     lg->GetIncludeDirectories(includePathList, target, lang, config, true);
     for (std::string const& i : includePathList) {
       ld.IncludePathList.push_back(
-        std::make_pair(i, target->IsSystemIncludeDirectory(i, config)));
+        std::make_pair(i, target->IsSystemIncludeDirectory(i, config, lang)));
     }
   }
 

Tests/IncludeDirectories/SystemIncludeDirectories/CMakeLists.txt

@@ -52,6 +52,13 @@ add_library(otherlib upstream.cpp)
 target_link_libraries(otherlib PUBLIC somelib)
 target_compile_options(somelib PRIVATE -Werror=unused-variable)
 
+add_library(iface_lang IMPORTED INTERFACE)
+set_property(TARGET iface_lang PROPERTY INTERFACE_INCLUDE_DIRECTORIES
+  "$<$<COMPILE_LANGUAGE:CXX>:${CMAKE_CURRENT_SOURCE_DIR}/systemlib_header_only>"
+  )
+add_library(imported_consumer_lang imported_consumer.cpp)
+target_link_libraries(imported_consumer_lang iface_lang)
+
 macro(do_try_compile error_option)
   set(TC_ARGS
     IFACE_TRY_COMPILE_${error_option}