Improve ATL/MFC string input support for C++17

Add support for any strings derived from CSimpleStringT when using C++17
compilers.

include/upa/str_arg.h

@@ -49,6 +49,12 @@ constexpr bool is_size_type_v =
     std::is_convertible_v<SizeT, std::size_t> ||
     std::is_convertible_v<SizeT, std::ptrdiff_t>;
 
+// See: https://en.cppreference.com/w/cpp/concepts/derived_from
+template<class Derived, class Base>
+constexpr bool is_derived_from_v =
+    std::is_base_of_v<Base, Derived> &&
+    std::is_convertible_v<const volatile Derived*, const volatile Base*>;
+
 
 // string args helper class
 

include/upa/url_for_atl.h

@@ -14,7 +14,7 @@
 #ifdef UPA_CPP_20
 # include <concepts>
 #else
-# include <cstringt.h>
+# include <type_traits>
 #endif
 
 namespace upa {
@@ -43,17 +43,14 @@ struct str_arg_char<StrT> : public str_arg_char_for_atl<StrT> {};
 
 #else // UPA_CPP_20
 
-template<typename CharT, bool mfcdll>
-struct str_arg_char<ATL::CSimpleStringT<CharT, mfcdll>> :
-    public str_arg_char_for_atl<ATL::CSimpleStringT<CharT, mfcdll>> {};
-
-template<typename CharT, class StringTraits>
-struct str_arg_char<ATL::CStringT<CharT, StringTraits>> :
-    public str_arg_char_for_atl<ATL::CStringT<CharT, StringTraits>> {};
-
-template<class StrT, int nChars>
-struct str_arg_char<ATL::CFixedStringT<StrT, nChars>> :
-    public str_arg_char_for_atl<ATL::CFixedStringT<StrT, nChars>> {};
+// CStringT and CFixedStringT are derived from CSimpleStringT
+template<class StrT>
+struct str_arg_char<StrT, std::enable_if_t<
+    is_derived_from_v<StrT, ATL::CSimpleStringT<char, true>> ||
+    is_derived_from_v<StrT, ATL::CSimpleStringT<wchar_t, true>> ||
+    is_derived_from_v<StrT, ATL::CSimpleStringT<char, false>> ||
+    is_derived_from_v<StrT, ATL::CSimpleStringT<wchar_t, false>>
+    >> : public str_arg_char_for_atl<StrT> {};
 
 #endif // UPA_CPP_20