Features: Add cxx_enum_forward_declarations.

Help/variable/CMAKE_CXX_KNOWN_FEATURES.rst

@@ -72,6 +72,11 @@ The features known to this version of CMake are:
 
   .. _N2346: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2346.htm
 
+``cxx_enum_forward_declarations``
+  Enum forward declarations, as defined in N2764_.
+
+  .. _N2764: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2764.pdf
+
 ``cxx_explicit_conversions``
   Explicit conversion operators, as defined in N2437_.
 

Modules/Compiler/GNU-CXX-FeatureTests.cmake

@@ -29,6 +29,7 @@ set(_cmake_feature_test_cxx_user_literals "${GNU47_CXX11}")
 # TODO: Should be supported by GNU 4.6
 set(GNU46_CXX11 "${_oldestSupported} && __cplusplus >= 201103L")
 set(_cmake_feature_test_cxx_constexpr "${GNU46_CXX11}")
+set(_cmake_feature_test_cxx_enum_forward_declarations "${GNU46_CXX11}")
 set(_cmake_feature_test_cxx_nullptr "${GNU46_CXX11}")
 set(_cmake_feature_test_cxx_range_for "${GNU46_CXX11}")
 set(_cmake_feature_test_cxx_unrestricted_unions "${GNU46_CXX11}")

Source/cmMakefile.cxx

@@ -54,6 +54,7 @@
   F(cxx_defaulted_functions) \
   F(cxx_delegating_constructors) \
   F(cxx_deleted_functions) \
+  F(cxx_enum_forward_declarations) \
   F(cxx_explicit_conversions) \
   F(cxx_extern_templates) \
   F(cxx_final) \

Tests/CompileFeatures/cxx_enum_forward_declarations.cpp

@@ -0,0 +1,8 @@
+
+enum SomeEnum : short;
+
+void someFunc()
+{
+  SomeEnum value;
+  int i = value;
+}