Add a variable to specify language-wide system include directories

Create a `CMAKE_<LANG>_STANDARD_INCLUDE_DIRECTORIES` variable to specify
system include directories for for `<LANG>` compiler command lines.
This plays a role for include directories as the existing
`CMAKE_<LANG>_STANDARD_LIBRARIES` variable does for link libraries.

Help/manual/cmake-variables.7.rst

@@ -365,6 +365,7 @@ Variables for Languages
    /variable/CMAKE_LANG_SIMULATE_VERSION
    /variable/CMAKE_LANG_SIZEOF_DATA_PTR
    /variable/CMAKE_LANG_SOURCE_FILE_EXTENSIONS
+   /variable/CMAKE_LANG_STANDARD_INCLUDE_DIRECTORIES
    /variable/CMAKE_LANG_STANDARD_LIBRARIES
    /variable/CMAKE_USER_MAKE_RULES_OVERRIDE_LANG
 

Help/release/dev/standard-include-directories.rst

@@ -0,0 +1,6 @@
+standard-include-directories
+----------------------------
+
+* A :variable:`CMAKE_<LANG>_STANDARD_INCLUDE_DIRECTORIES` variable was
+  added for use by toolchain files to specify system include directories
+  to be appended to all compiler command lines.

Help/variable/CMAKE_LANG_STANDARD_INCLUDE_DIRECTORIES.rst

@@ -0,0 +1,14 @@
+CMAKE_<LANG>_STANDARD_INCLUDE_DIRECTORIES
+-----------------------------------------
+
+Include directories to be used for every source file compiled with
+the ``<LANG>`` compiler.  This is meant for specification of system
+include directories needed by the language for the current platform.
+The directories always appear at the end of the include path passed
+to the compiler.
+
+This variable should not be set by project code.  It is meant to be set by
+CMake's platform information modules for the current toolchain, or by a
+toolchain file when used with :variable:`CMAKE_TOOLCHAIN_FILE`.
+
+See also :variable:`CMAKE_<LANG>_STANDARD_LIBRARIES`.

Help/variable/CMAKE_LANG_STANDARD_LIBRARIES.rst

@@ -8,3 +8,5 @@ libraries needed by the language for the current platform.
 This variable should not be set by project code.  It is meant to be set by
 CMake's platform information modules for the current toolchain, or by a
 toolchain file when used with :variable:`CMAKE_TOOLCHAIN_FILE`.
+
+See also :variable:`CMAKE_<LANG>_STANDARD_INCLUDE_DIRECTORIES`.

Source/cmGeneratorTarget.cxx

@@ -2415,6 +2415,18 @@ std::vector<std::string> cmGeneratorTarget::GetIncludeDirectories(
 
   cmDeleteAll(linkInterfaceIncludeDirectoriesEntries);
 
+  // Add standard include directories for this language.
+  std::string const standardIncludesVar =
+    "CMAKE_" + lang + "_STANDARD_INCLUDE_DIRECTORIES";
+  std::string const standardIncludes =
+    this->Makefile->GetSafeDefinition(standardIncludesVar);
+  std::vector<std::string>::size_type const before = includes.size();
+  cmSystemTools::ExpandListArgument(standardIncludes, includes);
+  for (std::vector<std::string>::iterator i = includes.begin() + before;
+       i != includes.end(); ++i) {
+    cmSystemTools::ConvertToUnixSlashes(*i);
+  }
+
   return includes;
 }
 

Source/cmGlobalGenerator.cxx

@@ -1385,6 +1385,9 @@ cmGlobalGenerator::CreateQtAutoGeneratorsTargets()
 
 void cmGlobalGenerator::FinalizeTargetCompileInfo()
 {
+  std::vector<std::string> const langs =
+    this->CMakeInstance->GetState()->GetEnabledLanguages();
+
   // Construct per-target generator information.
   for (unsigned int i = 0; i < this->Makefiles.size(); ++i) {
     cmMakefile* mf = this->Makefiles[i];
@@ -1429,6 +1432,23 @@ void cmGlobalGenerator::FinalizeTargetCompileInfo()
         }
       }
     }
+
+    // The standard include directories for each language
+    // should be treated as system include directories.
+    std::set<std::string> standardIncludesSet;
+    for (std::vector<std::string>::const_iterator li = langs.begin();
+         li != langs.end(); ++li) {
+      std::string const standardIncludesVar =
+        "CMAKE_" + *li + "_STANDARD_INCLUDE_DIRECTORIES";
+      std::string const standardIncludesStr =
+        mf->GetSafeDefinition(standardIncludesVar);
+      std::vector<std::string> standardIncludesVec;
+      cmSystemTools::ExpandListArgument(standardIncludesStr,
+                                        standardIncludesVec);
+      standardIncludesSet.insert(standardIncludesVec.begin(),
+                                 standardIncludesVec.end());
+    }
+    mf->AddSystemIncludeDirectories(standardIncludesSet);
   }
 }
 

Tests/IncludeDirectories/CMakeLists.txt

@@ -63,6 +63,7 @@ else()
     PROPERTIES COMPILE_FLAGS "-ITarProp")
 endif()
 
+add_subdirectory(StandardIncludeDirectories)
 add_subdirectory(TargetIncludeDirectories)
 
 set_property(DIRECTORY PROPERTY INCLUDE_DIRECTORIES "${CMAKE_BINARY_DIR}")

Tests/IncludeDirectories/StandardIncludeDirectories/CMakeLists.txt

@@ -0,0 +1,5 @@
+# Normally this variable should be set by a platform information module or
+# a toolchain file, but for purposes of this test we simply set it here.
+set(CMAKE_C_STANDARD_INCLUDE_DIRECTORIES ${CMAKE_CURRENT_SOURCE_DIR}/StdDir)
+
+add_executable(StandardIncludeDirectories main.c)

Tests/IncludeDirectories/StandardIncludeDirectories/main.c

@@ -0,0 +1,5 @@
+#include "StdIncDir.h"
+int main()
+{
+  return 0;
+}