Bug 826144 - Add MOZ_UTF16_HELPER() intermediary macro to handle UTF-16 stringification and concatenation correctly. r=waldo

--HG--
extra : rebase_source : 4c68df10e6e59af34db1e325232d4671492cc72f

mfbt/Char16.h

@@ -28,17 +28,27 @@
     * expressions (and pass char16_t pointers to Windows APIs). We #define our
     * char16_t as a macro to override yval.h's typedef of the same name.
     */
-#  define MOZ_UTF16(s) L##s
+#  define MOZ_UTF16_HELPER(s) L##s
 #  include <yvals.h>
 #  define char16_t wchar_t
 #elif defined(__cplusplus) && \
       (__cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__))
    /* C++11 has a builtin char16_t type. */
-#  define MOZ_UTF16(s) u##s
+#  define MOZ_UTF16_HELPER(s) u##s
 #else
 #  error "Char16.h requires C++11 (or something like it) for UTF-16 support."
 #endif
 
+/*
+ * Macro arguments used in concatenation or stringification won't be expanded.
+ * Therefore, in order for |MOZ_UTF16(FOO)| to work as expected (which is to
+ * expand |FOO| before doing whatever |MOZ_UTF16| needs to do to it) a helper
+ * macro, |MOZ_UTF16_HELPER| needs to be inserted in between to allow the macro
+ * argument to expand. See "3.10.6 Separate Expansion of Macro Arguments" of the
+ * CPP manual for a more accurate and precise explanation.
+ */
+#define MOZ_UTF16(s) MOZ_UTF16_HELPER(s)
+
 MOZ_STATIC_ASSERT(sizeof(char16_t) == 2, "Is char16_t type 16 bits?");
 MOZ_STATIC_ASSERT(sizeof(MOZ_UTF16('A')) == 2, "Is char literal 16 bits?");
 MOZ_STATIC_ASSERT(sizeof(MOZ_UTF16("")[0]) == 2, "Is string char 16 bits?");