Update cx::string to have better c++20 support

include/rexy/compat/cpp20/cx/string.hpp

@@ -0,0 +1,46 @@
+/**
+	This file is a part of rexy's general purpose library
+	Copyright (C) 2022 rexy712
+
+	This program is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 3 of the License, or
+	(at your option) any later version.
+
+	This program is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#ifndef REXY_COMPAT_CPP20_CX_STRING_HPP
+#define REXY_COMPAT_CPP20_CX_STRING_HPP
+
+#include "../../../string_base.hpp" //string_cat_expr
+#include "../../../string_view.hpp" //string_view
+#include <utility> //forward
+
+namespace rexy{
+	template<class T>
+	concept CxString = cx::is_cx_string<T>::value;
+}
+namespace rexy::cx{
+	template<CxString Str1, CxString Str2>
+	constexpr auto operator+(Str1&& l, Str2&& r)noexcept{
+		return string_cat_expr(std::forward<Str1>(l), std::forward<Str2>(r));
+	}
+	template<CxString Str1>
+	constexpr auto operator+(Str1&& l, const char* r)noexcept{
+		return string_cat_expr(std::forward<Str1>(l), rexy::basic_string_view<char>(r));
+	}
+	template<CxString Str1>
+	constexpr auto operator+(const char* l, Str1&& r)noexcept{
+		return string_cat_expr(rexy::basic_string_view<char>(l), std::forward<Str1>(r));
+	}
+
+}
+
+#endif