Add formatter<std::string_view>

include/fmt/format.h

@@ -3005,6 +3005,7 @@ FMT_FORMAT_AS(float, double);
 FMT_FORMAT_AS(Char*, const Char*);
 FMT_FORMAT_AS(std::basic_string<Char>, basic_string_view<Char>);
 FMT_FORMAT_AS(std::nullptr_t, const void*);
+FMT_FORMAT_AS(internal::std_string_view<Char>, basic_string_view<Char>);
 
 template <typename Char>
 struct formatter<void*, Char> : formatter<const void*, Char> {

test/format-test.cc

@@ -1590,9 +1590,20 @@ TEST(FormatterTest, FormatStringView) {
   EXPECT_EQ("", format("{}", string_view()));
 }
 
-#ifdef FMT_USE_STD_STRING_VIEW
+#ifdef FMT_USE_STRING_VIEW
+struct string_viewable {};
+
+FMT_BEGIN_NAMESPACE
+template <> struct formatter<string_viewable> : formatter<std::string_view> {
+  auto format(string_viewable, format_context& ctx) -> decltype(ctx.out()) {
+    return formatter<std::string_view>::format("foo", ctx);
+  }
+};
+FMT_END_NAMESPACE
+
 TEST(FormatterTest, FormatStdStringView) {
-  EXPECT_EQ("test", format("{0}", std::string_view("test")));
+  EXPECT_EQ("test", format("{}", std::string_view("test")));
+  EXPECT_EQ("foo", format("{}", string_viewable()));
 }
 #endif