Features: Make CMAKE_CXX_KNOWN_FEATURES a property.

As a 'built-in' variable it imposes a cost on all variable lookups
and it is expected to be rarely used.

Help/manual/cmake-properties.7.rst

@@ -16,6 +16,7 @@ Properties of Global Scope
    /prop_gbl/ALLOW_DUPLICATE_CUSTOM_TARGETS
    /prop_gbl/AUTOGEN_TARGETS_FOLDER
    /prop_gbl/AUTOMOC_TARGETS_FOLDER
+   /prop_gbl/CMAKE_CXX_KNOWN_FEATURES
    /prop_gbl/DEBUG_CONFIGURATIONS
    /prop_gbl/DISABLED_FEATURES
    /prop_gbl/ENABLED_FEATURES

Help/manual/cmake-variables.7.rst

@@ -258,7 +258,6 @@ Variables for Languages
 
    /variable/CMAKE_COMPILER_IS_GNULANG
    /variable/CMAKE_CXX_COMPILE_FEATURES
-   /variable/CMAKE_CXX_KNOWN_FEATURES
    /variable/CMAKE_CXX_STANDARD
    /variable/CMAKE_CXX_EXTENSIONS
    /variable/CMAKE_Fortran_MODDIR_DEFAULT

Help/prop_gbl/CMAKE_CXX_KNOWN_FEATURES.rst

@@ -3,7 +3,7 @@ CMAKE_CXX_KNOWN_FEATURES
 
 List of C++ features known to this version of CMake.
 
-The features listed in this variable may be known to be available to the
+The features listed in this global property may be known to be available to the
 C++ compiler.  If the feature is available with the C++ compiler, it will
 be listed in the :variable:`CMAKE_CXX_COMPILE_FEATURES` variable.
 

Help/variable/CMAKE_CXX_COMPILE_FEATURES.rst

@@ -4,5 +4,5 @@ CMAKE_CXX_COMPILE_FEATURES
 List of features known to the C++ compiler
 
 These features are known to be available for use with the C++ compiler. This
-list is a subset of the features listed in the :variable:`CMAKE_CXX_KNOWN_FEATURES`
-variable.
+list is a subset of the features listed in the :prop_gbl:`CMAKE_CXX_KNOWN_FEATURES`
+global property.

Modules/Internal/FeatureTesting.cmake

@@ -6,7 +6,10 @@ macro(record_compiler_features lang compile_flags feature_list)
   file(REMOVE "${CMAKE_BINARY_DIR}/CMakeFiles/feature_tests.bin")
   file(WRITE "${CMAKE_BINARY_DIR}/CMakeFiles/feature_tests.${lang_lc}" "
   extern const char features[] = {\"\"\n")
-  foreach(feature ${CMAKE_${lang}_KNOWN_FEATURES})
+
+  get_property(known_features GLOBAL PROPERTY CMAKE_${lang}_KNOWN_FEATURES)
+
+  foreach(feature ${known_features})
     if (_cmake_feature_test_${feature})
       if (${_cmake_feature_test_${feature}} STREQUAL 1)
         set(_feature_condition "\"1\" ")

Source/cmMakefile.cxx

@@ -41,54 +41,6 @@
 #include <ctype.h> // for isspace
 #include <assert.h>
 
-#define FOR_EACH_CXX_FEATURE(F) \
-  F(cxx_alias_templates) \
-  F(cxx_alignas) \
-  F(cxx_alignof) \
-  F(cxx_attributes) \
-  F(cxx_auto_type) \
-  F(cxx_constexpr) \
-  F(cxx_decltype) \
-  F(cxx_decltype_incomplete_return_types) \
-  F(cxx_default_function_template_args) \
-  F(cxx_defaulted_functions) \
-  F(cxx_defaulted_move_initializers) \
-  F(cxx_delegating_constructors) \
-  F(cxx_deleted_functions) \
-  F(cxx_enum_forward_declarations) \
-  F(cxx_explicit_conversions) \
-  F(cxx_extended_friend_declarations) \
-  F(cxx_extern_templates) \
-  F(cxx_final) \
-  F(cxx_func_identifier) \
-  F(cxx_generalized_initializers) \
-  F(cxx_inheriting_constructors) \
-  F(cxx_inline_namespaces) \
-  F(cxx_lambdas) \
-  F(cxx_local_type_template_args) \
-  F(cxx_long_long_type) \
-  F(cxx_noexcept) \
-  F(cxx_nonstatic_member_init) \
-  F(cxx_nullptr) \
-  F(cxx_override) \
-  F(cxx_range_for) \
-  F(cxx_raw_string_literals) \
-  F(cxx_reference_qualified_functions) \
-  F(cxx_right_angle_brackets) \
-  F(cxx_rvalue_references) \
-  F(cxx_sizeof_member) \
-  F(cxx_static_assert) \
-  F(cxx_strong_enums) \
-  F(cxx_template_template_parameters) \
-  F(cxx_thread_local) \
-  F(cxx_trailing_return_types) \
-  F(cxx_unicode_literals) \
-  F(cxx_uniform_initialization) \
-  F(cxx_unrestricted_unions) \
-  F(cxx_user_literals) \
-  F(cxx_variadic_macros) \
-  F(cxx_variadic_templates)
-
 class cmMakefile::Internals
 {
 public:
@@ -2499,12 +2451,6 @@ const char* cmMakefile::GetDefinition(const std::string& name) const
     {
     this->Internal->VarUsageStack.top().insert(name);
     }
-  if (name == "CMAKE_CXX_KNOWN_FEATURES")
-    {
-#define STRING_LIST_ELEMENT(F) ";" #F
-    return FOR_EACH_CXX_FEATURE(STRING_LIST_ELEMENT) + 1;
-#undef STRING_LIST_ELEMENT
-    }
   const char* def = this->Internal->VarStack.top().Get(name);
   if(!def)
     {

Source/cmake.cxx

@@ -2273,6 +2273,12 @@ const char *cmake::GetProperty(const std::string& prop,
       }
     this->SetProperty("ENABLED_LANGUAGES", lang.c_str());
     }
+  if (prop == "CMAKE_CXX_KNOWN_FEATURES")
+    {
+#define STRING_LIST_ELEMENT(F) ";" #F
+    return FOR_EACH_CXX_FEATURE(STRING_LIST_ELEMENT) + 1;
+#undef STRING_LIST_ELEMENT
+    }
   return this->Properties.GetPropertyValue(prop, scope, chain);
 }
 

Source/cmake.h

@@ -458,4 +458,52 @@ private:
   {"-Wno-dev", "Suppress developer warnings."},\
   {"-Wdev", "Enable developer warnings."}
 
+#define FOR_EACH_CXX_FEATURE(F) \
+  F(cxx_alias_templates) \
+  F(cxx_alignas) \
+  F(cxx_alignof) \
+  F(cxx_attributes) \
+  F(cxx_auto_type) \
+  F(cxx_constexpr) \
+  F(cxx_decltype) \
+  F(cxx_decltype_incomplete_return_types) \
+  F(cxx_default_function_template_args) \
+  F(cxx_defaulted_functions) \
+  F(cxx_defaulted_move_initializers) \
+  F(cxx_delegating_constructors) \
+  F(cxx_deleted_functions) \
+  F(cxx_enum_forward_declarations) \
+  F(cxx_explicit_conversions) \
+  F(cxx_extended_friend_declarations) \
+  F(cxx_extern_templates) \
+  F(cxx_final) \
+  F(cxx_func_identifier) \
+  F(cxx_generalized_initializers) \
+  F(cxx_inheriting_constructors) \
+  F(cxx_inline_namespaces) \
+  F(cxx_lambdas) \
+  F(cxx_local_type_template_args) \
+  F(cxx_long_long_type) \
+  F(cxx_noexcept) \
+  F(cxx_nonstatic_member_init) \
+  F(cxx_nullptr) \
+  F(cxx_override) \
+  F(cxx_range_for) \
+  F(cxx_raw_string_literals) \
+  F(cxx_reference_qualified_functions) \
+  F(cxx_right_angle_brackets) \
+  F(cxx_rvalue_references) \
+  F(cxx_sizeof_member) \
+  F(cxx_static_assert) \
+  F(cxx_strong_enums) \
+  F(cxx_template_template_parameters) \
+  F(cxx_thread_local) \
+  F(cxx_trailing_return_types) \
+  F(cxx_unicode_literals) \
+  F(cxx_uniform_initialization) \
+  F(cxx_unrestricted_unions) \
+  F(cxx_user_literals) \
+  F(cxx_variadic_macros) \
+  F(cxx_variadic_templates)
+
 #endif

Tests/CompileFeatures/CMakeLists.txt

@@ -22,7 +22,8 @@ macro(run_test feature)
   endif()
 endmacro()
 
-foreach(feature ${CMAKE_CXX_KNOWN_FEATURES})
+get_property(features GLOBAL PROPERTY CMAKE_CXX_KNOWN_FEATURES)
+foreach(feature ${features})
   run_test(${feature})
 endforeach()