cmGeneratorTarget: Include Cache now occurs per language+config

Previously only occurred per config which broke per-language
system includes.

Source/cmGeneratorTarget.cxx

@@ -1195,8 +1195,8 @@ bool cmGeneratorTarget::IsSystemIncludeDirectory(
     config_upper = cmSystemTools::UpperCase(config);
   }
 
-  using IncludeCacheType = std::map<std::string, std::vector<std::string>>;
-  auto iter = this->SystemIncludesCache.find(config_upper);
+  std::string key = cmStrCat(config_upper, "/", language);
+  auto iter = this->SystemIncludesCache.find(key);
 
   if (iter == this->SystemIncludesCache.end()) {
     cmGeneratorExpressionDAGChecker dagChecker(
@@ -1224,8 +1224,7 @@ bool cmGeneratorTarget::IsSystemIncludeDirectory(
     std::sort(result.begin(), result.end());
     result.erase(std::unique(result.begin(), result.end()), result.end());
 
-    IncludeCacheType::value_type entry(config_upper, result);
-    iter = this->SystemIncludesCache.insert(entry).first;
+    iter = this->SystemIncludesCache.emplace(key, result).first;
   }
 
   return std::binary_search(iter->second.begin(), iter->second.end(), dir);

Source/cmGeneratorTarget.h

@@ -849,6 +849,8 @@ private:
   mutable std::set<std::string> VisitedConfigsForObjects;
   mutable std::map<cmSourceFile const*, std::string> Objects;
   std::set<cmSourceFile const*> ExplicitObjectName;
+
+  // "config/language" is the key
   mutable std::map<std::string, std::vector<std::string>> SystemIncludesCache;
 
   mutable std::string ExportMacro;

Tests/IncludeDirectories/CMakeLists.txt

@@ -17,6 +17,7 @@ if (((CMAKE_C_COMPILER_ID STREQUAL GNU AND CMAKE_C_COMPILER_VERSION VERSION_GREA
   endif()
   if (run_sys_includes_test)
     add_subdirectory(SystemIncludeDirectories)
+    add_subdirectory(SystemIncludeDirectoriesPerLang)
   endif()
 endif()
 

Tests/IncludeDirectories/SystemIncludeDirectoriesPerLang/CMakeLists.txt

@@ -0,0 +1,23 @@
+cmake_minimum_required(VERSION 3.17 FATAL_ERROR)
+
+project(SystemIncludeDirectoriesPerLang)
+
+add_library(c_interface INTERFACE)
+set_target_properties(c_interface PROPERTIES
+  INTERFACE_INCLUDE_DIRECTORIES "$<$<COMPILE_LANGUAGE:C>:${CMAKE_CURRENT_SOURCE_DIR}>"
+  INTERFACE_SYSTEM_INCLUDE_DIRECTORIES "$<$<COMPILE_LANGUAGE:C>:${CMAKE_CURRENT_SOURCE_DIR}>"
+)
+target_compile_options(c_interface INTERFACE "$<$<COMPILE_LANG_AND_ID:C,GNU,Clang>:-Werror=unused-variable>")
+
+add_library(cxx_interface INTERFACE)
+set_target_properties(cxx_interface PROPERTIES
+  INTERFACE_INCLUDE_DIRECTORIES "$<$<COMPILE_LANGUAGE:CXX>:${CMAKE_CURRENT_SOURCE_DIR}/cxx_system_include>"
+  INTERFACE_SYSTEM_INCLUDE_DIRECTORIES "$<$<COMPILE_LANGUAGE:CXX>:${CMAKE_CURRENT_SOURCE_DIR}/cxx_system_include>"
+)
+target_compile_options(cxx_interface INTERFACE "$<$<COMPILE_LANG_AND_ID:CXX,GNU,Clang>:-Werror=unused-variable>")
+
+# The C header must come before the C++ header for this test to smoke out the
+# failure. The order of sources is how CMake determines the include cache
+# and we need it to cache on the 'bad' language first
+add_executable(consume_multi_lang_includes main.c smoke_out_includes.cxx)
+target_link_libraries(consume_multi_lang_includes PRIVATE c_interface cxx_interface)

Tests/IncludeDirectories/SystemIncludeDirectoriesPerLang/cxx_system_include/header.h

@@ -0,0 +1,10 @@
+
+// Generate a warning in here
+
+int function_that_generates_warning(int x)
+{
+  int y = x;
+  int z = 2;
+  y -= x;
+  return y;
+}

Tests/IncludeDirectories/SystemIncludeDirectoriesPerLang/main.c

@@ -0,0 +1,4 @@
+int main()
+{
+  return 0;
+}

Tests/IncludeDirectories/SystemIncludeDirectoriesPerLang/smoke_out_includes.cxx

@@ -0,0 +1,7 @@
+
+#include <header.h>
+
+int empty_func()
+{
+  return function_that_generates_warning(4);
+}