Added BOOST_MAY_ALIAS and BOOST_NO_MAY_ALIAS macros.

The macros can be used to mark types that can alias other types (i.e. break
C++ strict aliasing rules).

doc/macro_reference.qbk

@@ -1262,6 +1262,18 @@ Usage example:
 ]]
 [[`BOOST_ATTRIBUTE_UNUSED`][Expands to `__attribute__((unused))` when this is available - 
 can be used to disable compiler warnings relating to unused types or variables.]]
+[[`BOOST_MAY_ALIAS`, `BOOST_NO_MAY_ALIAS`][
+`BOOST_MAY_ALIAS` expands to a type attribute that can be used to mark types that may
+alias other types. Pointers or references to such marked types can be used to access objects
+of other types. If the compiler supports this feature `BOOST_NO_MAY_ALIAS` is not defined.
+Otherwise `BOOST_MAY_ALIAS` expands to nothing and `BOOST_NO_MAY_ALIAS` is defined.
+
+Usage example:
+``
+  struct BOOST_MAY_ALIAS aliasing_struct;
+  typedef unsigned int BOOST_MAY_ALIAS aliasing_uint;
+``
+]]
 ]
 
 [endsect]

include/boost/config/compiler/clang.hpp

@@ -318,6 +318,11 @@
 // Clang has supported the 'unused' attribute since the first release.
 #define BOOST_ATTRIBUTE_UNUSED __attribute__((__unused__))
 
+// Type aliasing hint.
+#if __has_attribute(__may_alias__)
+#  define BOOST_MAY_ALIAS __attribute__((__may_alias__))
+#endif
+
 #ifndef BOOST_COMPILER
 #  define BOOST_COMPILER "Clang version " __clang_version__
 #endif

include/boost/config/compiler/gcc.hpp

@@ -315,6 +315,10 @@
 #if __GNUC__ >= 4
 #  define BOOST_ATTRIBUTE_UNUSED __attribute__((__unused__))
 #endif
+
+// Type aliasing hint. Supported since gcc 3.3.
+#define BOOST_MAY_ALIAS __attribute__((__may_alias__))
+
 //
 // __builtin_unreachable:
 #if BOOST_GCC_VERSION >= 40800

include/boost/config/compiler/intel.hpp

@@ -311,6 +311,12 @@ template<> struct assert_intrinsic_wchar_t<unsigned short> {};
 #  define BOOST_SYMBOL_IMPORT
 #  define BOOST_SYMBOL_VISIBLE __attribute__((visibility("default")))
 #endif
+
+// Type aliasing hint
+#if defined(__GNUC__) && (BOOST_INTEL_CXX_VERSION >= 1300)
+#  define BOOST_MAY_ALIAS __attribute__((__may_alias__))
+#endif
+
 //
 // C++0x features
 // For each feature we need to check both the Intel compiler version, 

include/boost/config/compiler/vacpp.hpp

@@ -65,6 +65,11 @@
 #define BOOST_NO_PARTIAL_SPECIALIZATION_IMPLICIT_DEFAULT_ARGS
 #endif
 
+// Type aliasing hint. Supported since XL C++ 13.1
+#if (__IBMCPP__ >= 1310)
+#  define BOOST_MAY_ALIAS __attribute__((__may_alias__))
+#endif
+
 //
 // C++0x features
 //

include/boost/config/compiler/xlcpp.hpp

@@ -267,6 +267,11 @@
 #  define BOOST_ATTRIBUTE_UNUSED __attribute__((unused))
 #endif
 
+// Type aliasing hint.
+#if __has_attribute(__may_alias__)
+#  define BOOST_MAY_ALIAS __attribute__((__may_alias__))
+#endif
+
 #ifndef BOOST_COMPILER
 #  define BOOST_COMPILER "Clang version " __clang_version__
 #endif

include/boost/config/compiler/xlcpp_zos.hpp

@@ -159,6 +159,7 @@
 #if defined(__IBM_ATTRIBUTES)
 #  define BOOST_FORCEINLINE inline __attribute__ ((__always_inline__))
 #  define BOOST_NOINLINE __attribute__ ((__noinline__))
+#  define BOOST_MAY_ALIAS __attribute__((__may_alias__))
 // No BOOST_ALIGNMENT - explicit alignment support is broken (V2R1).
 #endif
 

include/boost/config/detail/suffix.hpp

@@ -602,6 +602,14 @@ namespace std{ using ::type_info; }
 #  endif
 #endif
 
+// BOOST_MAY_ALIAS -----------------------------------------------//
+// The macro expands to an attribute to mark a type that is allowed to alias other types.
+// The macro is defined in the compiler-specific headers.
+#if !defined(BOOST_MAY_ALIAS)
+#  define BOOST_NO_MAY_ALIAS
+#  define BOOST_MAY_ALIAS
+#endif
+
 // BOOST_FORCEINLINE ---------------------------------------------//
 // Macro to use in place of 'inline' to force a function to be inline
 #if !defined(BOOST_FORCEINLINE)

test/config_info.cpp

@@ -1152,6 +1152,8 @@ void print_boost_macros()
    PRINT_MACRO(BOOST_FORCEINLINE);
    PRINT_MACRO(BOOST_NOINLINE);
    PRINT_MACRO(BOOST_FALLTHROUGH);
+   PRINT_MACRO(BOOST_MAY_ALIAS);
+   PRINT_MACRO(BOOST_NO_MAY_ALIAS);
 }
 
 void print_separator()

test/helper_macro_test.cpp

@@ -34,6 +34,9 @@ BOOST_NORETURN void always_throw()
    throw 0;
 }
 
+struct BOOST_MAY_ALIAS aliasing_struct {};
+typedef unsigned int BOOST_MAY_ALIAS aliasing_uint;
+
 
 #define test_fallthrough(x) foobar(x)