Features: Add cxx_alignas.

cxx_alignof will be a separate feature known to CMake, because
both can be implemented with separate backward compatibility
versions.

Help/variable/CMAKE_CXX_KNOWN_FEATURES.rst

@@ -17,6 +17,11 @@ The features known to this version of CMake are:
 
   .. _N2258: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2258.pdf
 
+``cxx_alignas``
+  Alignment control ``alignas``, as defined in N2341_.
+
+  .. _N2341: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2341.pdf
+
 ``cxx_auto_type``
   Automatic type deduction, as defined in N1984_.
 

Modules/Compiler/GNU-CXX-FeatureTests.cmake

@@ -6,6 +6,7 @@ set(_oldestSupported "(__GNUC__ * 100 + __GNUC_MINOR__) >= 408")
 set(_cmake_feature_test_cxx_reference_qualified_functions
   "((__GNUC__ * 100 + __GNUC_MINOR__) > 408 || __GNUC_PATCHLEVEL__ >= 1) && __cplusplus >= 201103L")
 set(GNU48_CXX11 "(__GNUC__ * 100 + __GNUC_MINOR__) >= 408 && __cplusplus >= 201103L")
+set(_cmake_feature_test_cxx_alignas "${GNU48_CXX11}")
 set(_cmake_feature_test_cxx_inheriting_constructors "${GNU48_CXX11}")
 # TODO: Should be supported by GNU 4.7
 set(GNU47_CXX11 "${_oldestSupported} && __cplusplus >= 201103L")

Source/cmMakefile.cxx

@@ -43,6 +43,7 @@
 
 #define FOR_EACH_CXX_FEATURE(F) \
   F(cxx_alias_templates) \
+  F(cxx_alignas) \
   F(cxx_auto_type) \
   F(cxx_constexpr) \
   F(cxx_decltype) \

Tests/CompileFeatures/cxx_alignas.cpp

@@ -0,0 +1,4 @@
+
+struct S1 {
+  alignas(8) int n;
+};