[WIP] Loosen alphabet_variant requirement

Author: eseiler

include/seqan3/alphabet/composite/alphabet_variant.hpp

@@ -121,18 +121,22 @@ template <typename... alternative_types>
     requires (detail::writable_constexpr_alphabet<alternative_types> && ...) && (std::regular<alternative_types> && ...)
           && (sizeof...(alternative_types) >= 2)
 class alphabet_variant :
-    public alphabet_base<alphabet_variant<alternative_types...>,
-                         (static_cast<size_t>(alphabet_size<alternative_types>) + ...),
-                         char>
+    public alphabet_base<
+        alphabet_variant<alternative_types...>,
+        (static_cast<size_t>(alphabet_size<alternative_types>) + ...),
+        std::conditional_t<(std::same_as<alphabet_char_t<alternative_types>, char> && ...), char, char16_t>>
 {
 private:
     //!\brief The base type.
-    using base_t = alphabet_base<alphabet_variant<alternative_types...>,
-                                 (static_cast<size_t>(alphabet_size<alternative_types>) + ...),
-                                 char>;
-
-    static_assert((std::is_same_v<alphabet_char_t<alternative_types>, char> && ...),
-                  "The alphabet_variant is currently only tested for alphabets with char_type char. "
+    using base_t = alphabet_base<
+        alphabet_variant<alternative_types...>,
+        (static_cast<size_t>(alphabet_size<alternative_types>) + ...),
+        std::conditional_t<(std::same_as<alphabet_char_t<alternative_types>, char> && ...), char, char16_t>>;
+
+    static_assert(((std::is_same_v<alphabet_char_t<alternative_types>, char>
+                    || std::is_same_v<alphabet_char_t<alternative_types>, char16_t>)
+                   && ...),
+                  "The alphabet_variant is currently only tested for alphabets with char_type char or char16_t. "
                   "Contact us on GitHub if you have a different use case: https://github.com/seqan/seqan3 .");
 
     //!\brief Befriend the base type.