Genex: Fix COMPILE_LANGUAGE propagation through try_compile

When evaluating include directories during export to a `try_compile`
test project, thread the compile language through to the generator
expression evaluator so it can support `$<COMPILE_LANGUAGE:...>`.

Issue: #17811

Source/cmCoreTryCompile.cxx

@@ -580,7 +580,8 @@ int cmCoreTryCompile::TryCompileCode(std::vector<std::string> const& argv,
 
     if (!targets.empty()) {
       std::string fname = "/" + std::string(targetName) + "Targets.cmake";
-      cmExportTryCompileFileGenerator tcfg(gg, targets, this->Makefile);
+      cmExportTryCompileFileGenerator tcfg(gg, targets, this->Makefile,
+                                           testLangs);
       tcfg.SetExportFile((this->BinaryDirectory + fname).c_str());
       tcfg.SetConfig(tcConfig);
 

Source/cmExportTryCompileFileGenerator.cxx

@@ -18,7 +18,8 @@
 
 cmExportTryCompileFileGenerator::cmExportTryCompileFileGenerator(
   cmGlobalGenerator* gg, const std::vector<std::string>& targets,
-  cmMakefile* mf)
+  cmMakefile* mf, std::set<std::string> const& langs)
+  : Languages(langs.begin(), langs.end())
 {
   gg->CreateImportedGenerationObjects(mf, targets, this->Exports);
 }
@@ -36,12 +37,14 @@ bool cmExportTryCompileFileGenerator::GenerateMainFile(std::ostream& os)
 
       ImportPropertyMap properties;
 
+      for (std::string const& lang : this->Languages) {
 #define FIND_TARGETS(PROPERTY)                                                \
-  this->FindTargets("INTERFACE_" #PROPERTY, te, emittedDeps);
+  this->FindTargets("INTERFACE_" #PROPERTY, te, lang, emittedDeps);
 
-      CM_FOR_EACH_TRANSITIVE_PROPERTY_NAME(FIND_TARGETS)
+        CM_FOR_EACH_TRANSITIVE_PROPERTY_NAME(FIND_TARGETS)
 
 #undef FIND_TARGETS
+      }
 
       this->PopulateProperties(te, properties, emittedDeps);
 
@@ -53,7 +56,7 @@ bool cmExportTryCompileFileGenerator::GenerateMainFile(std::ostream& os)
 
 std::string cmExportTryCompileFileGenerator::FindTargets(
   const std::string& propName, cmGeneratorTarget const* tgt,
-  std::set<cmGeneratorTarget const*>& emitted)
+  std::string const& language, std::set<cmGeneratorTarget const*>& emitted)
 {
   const char* prop = tgt->GetProperty(propName);
   if (!prop) {
@@ -72,8 +75,9 @@ std::string cmExportTryCompileFileGenerator::FindTargets(
 
   cmGeneratorTarget gDummyHead(&dummyHead, tgt->GetLocalGenerator());
 
-  std::string result = cge->Evaluate(tgt->GetLocalGenerator(), this->Config,
-                                     false, &gDummyHead, tgt, &dagChecker);
+  std::string result =
+    cge->Evaluate(tgt->GetLocalGenerator(), this->Config, false, &gDummyHead,
+                  tgt, &dagChecker, language);
 
   const std::set<cmGeneratorTarget const*>& allTargets =
     cge->GetAllTargetsSeen();
@@ -97,7 +101,8 @@ void cmExportTryCompileFileGenerator::PopulateProperties(
     if (p.find("IMPORTED_LINK_INTERFACE_LIBRARIES") == 0 ||
         p.find("IMPORTED_LINK_DEPENDENT_LIBRARIES") == 0 ||
         p.find("INTERFACE_LINK_LIBRARIES") == 0) {
-      std::string evalResult = this->FindTargets(p, target, emitted);
+      std::string evalResult =
+        this->FindTargets(p, target, std::string(), emitted);
 
       std::vector<std::string> depends;
       cmSystemTools::ExpandListArgument(evalResult, depends);

Source/cmExportTryCompileFileGenerator.h

@@ -21,7 +21,8 @@ class cmExportTryCompileFileGenerator : public cmExportFileGenerator
 public:
   cmExportTryCompileFileGenerator(cmGlobalGenerator* gg,
                                   std::vector<std::string> const& targets,
-                                  cmMakefile* mf);
+                                  cmMakefile* mf,
+                                  std::set<std::string> const& langs);
 
   /** Set the list of targets to export.  */
   void SetConfig(const std::string& config) { this->Config = config; }
@@ -49,10 +50,12 @@ protected:
 private:
   std::string FindTargets(const std::string& prop,
                           const cmGeneratorTarget* tgt,
+                          std::string const& language,
                           std::set<const cmGeneratorTarget*>& emitted);
 
   std::vector<cmGeneratorTarget const*> Exports;
   std::string Config;
+  std::vector<std::string> Languages;
 };
 
 #endif

Tests/IncludeDirectories/SystemIncludeDirectories/CMakeLists.txt

@@ -32,7 +32,9 @@ target_link_libraries(consumer upstream config_specific)
 target_compile_options(consumer PRIVATE -Werror=unused-variable)
 
 add_library(iface IMPORTED INTERFACE)
-set_property(TARGET iface PROPERTY INTERFACE_INCLUDE_DIRECTORIES "${CMAKE_CURRENT_SOURCE_DIR}/systemlib_header_only")
+set_property(TARGET iface PROPERTY INTERFACE_INCLUDE_DIRECTORIES
+  "$<$<COMPILE_LANGUAGE:CXX>:${CMAKE_CURRENT_SOURCE_DIR}/systemlib_header_only>"
+  )
 
 add_library(imported_consumer imported_consumer.cpp)
 target_link_libraries(imported_consumer iface)
@@ -52,13 +54,6 @@ 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}