string_view::char_type -> value_type (#1539)

include/fmt/core.h

@@ -313,7 +313,8 @@ template <typename Char> class basic_string_view {
   size_t size_;
 
  public:
-  using char_type = Char;
+  using char_type FMT_DEPRECATED_ALIAS = Char;
+  using value_type = Char;
   using iterator = const Char*;
 
   FMT_CONSTEXPR basic_string_view() FMT_NOEXCEPT : data_(nullptr), size_(0) {}
@@ -471,7 +472,7 @@ struct is_string : std::is_class<decltype(to_string_view(std::declval<S>()))> {
 template <typename S, typename = void> struct char_t_impl {};
 template <typename S> struct char_t_impl<S, enable_if_t<is_string<S>::value>> {
   using result = decltype(to_string_view(std::declval<S>()));
-  using type = typename result::char_type;
+  using type = typename result::value_type;
 };
 
 struct error_handler {

test/core-test.cc

@@ -402,8 +402,12 @@ TEST(ArgTest, VisitInvalidArg) {
   fmt::visit_format_arg(visitor, arg);
 }
 
+TEST(StringViewTest, ValueType) {
+  static_assert(std::is_same<string_view::value_type, char>::value, "");
+}
+
 TEST(StringViewTest, Length) {
-  // Test that StringRef::size() returns string length, not buffer size.
+  // Test that string_view::size() returns string length, not buffer size.
   char str[100] = "some string";
   EXPECT_EQ(std::strlen(str), string_view(str).size());
   EXPECT_LT(std::strlen(str), sizeof(str));