Fix formatting of objects convertible to string_view

include/fmt/core.h

@@ -570,7 +570,8 @@ FMT_MAKE_VALUE(cstring_type, signed char*, const signed char*)
 FMT_MAKE_VALUE(cstring_type, const signed char*, const signed char*)
 FMT_MAKE_VALUE(cstring_type, unsigned char*, const unsigned char*)
 FMT_MAKE_VALUE(cstring_type, const unsigned char*, const unsigned char*)
-FMT_MAKE_VALUE(string_type, basic_string_view<Char>, basic_string_view<Char>)
+FMT_MAKE_VALUE(string_type, basic_string_view<typename C::char_type>,
+               basic_string_view<Char>)
 FMT_MAKE_VALUE(string_type, const std::basic_string<typename C::char_type>&,
                basic_string_view<Char>)
 FMT_MAKE_VALUE(pointer_type, void*, const void*)
@@ -595,6 +596,7 @@ inline typename std::enable_if<
 template <typename C, typename T, typename Char = typename C::char_type>
 inline typename std::enable_if<
     !convert_to_int<T, Char>::value &&
+    !std::is_convertible<T, basic_string_view<Char>>::value &&
     !std::is_convertible<T, std::basic_string<Char>>::value,
     typed_value<C, custom_type>>::type
   make_value(const T &val) { return val; }

test/format-test.cc

@@ -1223,6 +1223,14 @@ TEST(FormatterTest, FormatConvertibleToString) {
   EXPECT_EQ("foo", format("{}", ConvertibleToString()));
 }
 
+struct ConvertibleToStringView {
+  operator fmt::string_view() const { return "foo"; }
+};
+
+TEST(FormatterTest, FormatConvertibleToStringView) {
+  EXPECT_EQ("foo", format("{}", ConvertibleToStringView()));
+}
+
 namespace fmt {
 template <>
 struct formatter<Date> {