compat: add a simple std::ctype<char16_t> for OSX

Standard C++ implementation has std::ctype<wchar_t> for UTF-32, but no
UTF-16 one. Since NSString is UTF-16, we need support for UTF-16 in C++
too if unnecessary translations between UTF-16/32 are to be avoided.

Signed-off-by: Simon Rozman <simon@rozman.si>

include/stdex/compat.hpp

@@ -10,7 +10,10 @@
 #include "windows.h"
 #include <sal.h>
 #include <tchar.h>
+#else
+#include <uchar.h>
 #endif
+#include <locale>
 #include <type_traits>
 
 #ifndef _In_
@@ -223,3 +226,108 @@ size_t _countof(const T (&arr)[N])
 #define lockf64 lockf
 #define ftruncate64 ftruncate
 #endif
+
+#ifndef _WIN32
+// On Windows, wchar_t is UTF-16. On POSIX it is UTF32, and we are left out of UTF16 support in std C++.
+
+namespace std
+{
+	///
+	/// This is a poor man's instantiation of a `std::ctype<char16_t>`.
+	///
+	template <>
+	class ctype<char16_t> : public locale::facet, public ctype_base
+	{
+	public:
+		typedef char16_t char_type;
+
+		explicit ctype(size_t refs = 0) :
+			locale::facet(refs),
+			m_ctypew(std::use_facet<std::ctype<wchar_t>>(std::locale()))
+		{}
+
+		bool is(mask m, char_type c) const
+		{
+			return do_is(m, c);
+		}
+
+		const char_type* is(const char_type* low, const char_type* high, mask* vec) const
+		{
+			return do_is(low, high, vec);
+		}
+
+		const char_type* scan_is(mask m, const char_type* low, const char_type* high) const
+		{
+			return do_scan_is(m, low, high);
+		}
+
+		const char_type* scan_not(mask m, const char_type* low, const char_type* high) const
+		{
+			return do_scan_not(m, low, high);
+		}
+
+		char_type toupper(char_type c) const
+		{
+			return do_toupper(c);
+		}
+
+		const char_type* toupper(char_type* low, const char_type* high) const
+		{
+			return do_toupper(low, high);
+		}
+
+		char_type tolower(char_type c) const
+		{
+			return do_tolower(c);
+		}
+
+		const char_type* tolower(char_type* low, const char_type* high) const
+		{
+			return do_tolower(low, high);
+		}
+
+		char_type widen(char c) const
+		{
+			return do_widen(c);
+		}
+
+		const char* widen(const char* low, const char* high, char_type* to) const
+		{
+			return do_widen(low, high, to);
+		}
+
+		char narrow(char_type c, char dfault) const
+		{
+			return do_narrow(c, dfault);
+		}
+
+		const char_type* narrow(const char_type* low, const char_type* high, char dfault, char* to) const
+		{
+			return do_narrow(low, high, dfault, to);
+		}
+
+		static inline locale::id id;
+
+	protected:
+		const std::ctype<wchar_t> &m_ctypew;
+
+		bool do_is(mask m, char_type c) const { return m_ctypew.is(m, c); }
+
+		const char_type* do_is(const char_type* low, const char_type* high, mask* vec) const;
+		const char_type* do_scan_is(mask m, const char_type* low, const char_type* high) const;
+		const char_type* do_scan_not(mask m, const char_type* low, const char_type* high) const;
+
+		char_type do_toupper(char_type c) const { return m_ctypew.toupper(c); }
+
+		const char_type* do_toupper(char_type* low, const char_type* high) const;
+
+		char_type do_tolower(char_type c) const { return m_ctypew.tolower(c); }
+
+		const char_type* do_tolower(char_type* low, const char_type* high) const;
+		char_type do_widen(char) const;
+		const char* do_widen(const char* low, const char* high, char_type* dest) const;
+		char do_narrow(char_type, char dfault) const;
+		const char_type* do_narrow(const char_type* low, const char_type* high, char dfault, char* dest) const;
+	};
+}
+#endif