Provide reasonable default definition for is_contiguous

Author: user202729

include/fmt/base.h

@@ -588,7 +588,15 @@ template <typename T> class basic_appender;
 using appender = basic_appender<char>;
 
 // Checks whether T is a container with contiguous storage.
-template <typename T> struct is_contiguous : std::false_type {};
+template <typename T, typename = void>
+struct is_contiguous : std::false_type {};
+
+template <typename T>
+struct is_contiguous<T, void_t<decltype(std::declval<T&>().data()),
+                               decltype(std::declval<T&>().size()),
+                               decltype(std::declval<T&>().resize(size_t{})),
+                               decltype(std::declval<T&>()[size_t{}])>>
+    : std::true_type {};
 
 class context;
 template <typename OutputIt, typename Char> class generic_context;