Provide reasonable default definition for is_contiguous

include/fmt/base.h

@@ -593,7 +593,20 @@ 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 {};
+namespace detail {
+
+template <typename T, typename Enable = 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&>()[size_t{}])>>
+    : std::true_type {};
+
+}  // namespace detail
+
+template <typename T> struct is_contiguous : detail::is_contiguous_<T> {};
 
 class context;
 template <typename OutputIt, typename Char> class generic_context;

test/base-test.cc

@@ -21,8 +21,10 @@
 #include <functional>   // std::equal_to
 #include <iterator>     // std::back_insert_iterator, std::distance
 #include <limits>       // std::numeric_limits
+#include <list>         // std::list
 #include <string>       // std::string
 #include <type_traits>  // std::is_same
+#include <vector>       // std::vector
 
 #include "gmock/gmock.h"
 
@@ -876,6 +878,14 @@ FMT_BEGIN_NAMESPACE
 template <> struct is_contiguous<custom_container> : std::true_type {};
 FMT_END_NAMESPACE
 
+TEST(base_test, is_contiguous) {
+  EXPECT_TRUE((fmt::is_contiguous<custom_container>::value));
+  EXPECT_TRUE((fmt::is_contiguous<std::string>::value));
+  EXPECT_TRUE((fmt::is_contiguous<fmt::string_view>::value));
+  EXPECT_TRUE((fmt::is_contiguous<std::vector<char>>::value));
+  EXPECT_FALSE((fmt::is_contiguous<std::list<char>>::value));
+}
+
 TEST(base_test, format_to_custom_container) {
   auto c = custom_container();
   fmt::format_to(std::back_inserter(c), "");