Add mbed::byte as per C++17 std::byte

Author: kjbracey

platform/mbed_cxxsupport.h

@@ -737,6 +737,87 @@ template<class T>
 void as_const(T &&) = delete;
 #endif
 
+/* C++17 byte */
+#if __cplusplus >= 201703 || __cpp_lib_byte >= 201603
+using std::byte;
+#else
+enum class byte : unsigned char {};
+
+template <class IntType>
+constexpr std::enable_if_t<std::is_integral<IntType>::value,
+byte &> operator<<=(byte &b, IntType shift) noexcept
+{
+    return b = byte(static_cast<unsigned char>(b) << shift);
+}
+
+template <class IntType>
+constexpr std::enable_if_t<std::is_integral<IntType>::value,
+byte> operator<<(byte b, IntType shift) noexcept
+{
+    return byte(static_cast<unsigned char>(b) << shift);
+}
+
+template <class IntType>
+constexpr std::enable_if_t<std::is_integral<IntType>::value,
+byte &> operator>>=(byte &b, IntType shift) noexcept {
+    return b = byte(static_cast<unsigned char>(b) >> shift);
+}
+
+template <class IntType>
+constexpr std::enable_if_t<std::is_integral<IntType>::value,
+byte> operator>>(byte b, IntType shift) noexcept
+{
+    return byte(static_cast<unsigned char>(b) >> shift);
+}
+
+template <class IntType>
+constexpr byte &operator|=(byte &l, byte r) noexcept
+{
+    return l = byte(static_cast<unsigned char>(l) | static_cast<unsigned char>(r));
+}
+
+template <class IntType>
+constexpr byte operator|(byte &l, byte r) noexcept {
+    return byte(static_cast<unsigned char>(l) | static_cast<unsigned char>(r));
+}
+
+template <class IntType>
+constexpr byte &operator&=(byte &l, byte r) noexcept
+{
+    return l = byte(static_cast<unsigned char>(l) & static_cast<unsigned char>(r));
+}
+
+template <class IntType>
+constexpr byte operator&(byte &l, byte r) noexcept
+{
+    return byte(static_cast<unsigned char>(l) & static_cast<unsigned char>(r));
+}
+
+template <class IntType>
+constexpr byte &operator^=(byte &l, byte r) noexcept
+{
+    return l = byte(static_cast<unsigned char>(l) ^ static_cast<unsigned char>(r));
+}
+
+template <class IntType>
+constexpr byte operator^(byte &l, byte r) noexcept
+{
+    return byte(static_cast<unsigned char>(l) ^ static_cast<unsigned char>(r));
+}
+
+template <class IntType>
+constexpr byte operator~(byte b) noexcept
+{
+    return byte(~static_cast<unsigned char>(b));
+}
+
+template <class IntType>
+constexpr std::enable_if_t<std::is_integral<IntType>::value,
+IntType> to_integer(byte b) noexcept
+{
+    return IntType(b);
+}
+#endif
 } // namespace mbed
 
 #endif // MBED_CXXSUPPORT_H