Provide reasonable default definition for is_contiguous

Author: user202729

include/fmt/base.h

@@ -87,6 +87,10 @@
 #  define FMT_HAS_CPP_ATTRIBUTE(x) 0
 #endif
 
+#if FMT_HAS_INCLUDE(<iterator>)
+#  include <iterator>
+#endif
+
 #define FMT_HAS_CPP14_ATTRIBUTE(attribute) \
   (FMT_CPLUSPLUS >= 201402L && FMT_HAS_CPP_ATTRIBUTE(attribute))
 
@@ -588,7 +592,21 @@ template <typename T> class basic_appender;
 using appender = basic_appender<char>;
 
 // Checks whether T is a container with contiguous storage.
+#if FMT_HAS_INCLUDE(<iterator>)
+template <typename T, typename = void>
+struct is_contiguous : std::false_type {};
+
+template <typename T>
+struct is_contiguous<
+    T,
+    void_t<decltype(std::data(std::declval<T&>())),
+           decltype(std::declval<T&>().size()),
+           decltype(std::declval<T&>().resize(size_t{})),
+           decltype(std::declval<T&>()[size_t{}])>>
+    : std::true_type {};
+#else
 template <typename T> struct is_contiguous : std::false_type {};
+#endif
 
 class context;
 template <typename OutputIt, typename Char> class generic_context;