Enable inheriting from formatter<std::string_view>

Fixes fmtlib#4036
This formatter specialization with base::format means a class implicitly
convertible to std::string_view will now be converted by this format
function before being passed to the fmt::string_view format function.
This wouldn't work previously as the compiler may only perform one
implicit conversion, and we need 2 here (from our type, to
std::string_view, to fmt::string_view).

include/fmt/format.h

@@ -3990,13 +3990,23 @@ FMT_FORMAT_AS(long, detail::long_type);
 FMT_FORMAT_AS(unsigned long, detail::ulong_type);
 FMT_FORMAT_AS(Char*, const Char*);
 FMT_FORMAT_AS(std::nullptr_t, const void*);
-FMT_FORMAT_AS(detail::std_string_view<Char>, basic_string_view<Char>);
 FMT_FORMAT_AS(void*, const void*);
 
 template <typename Char, typename Traits, typename Allocator>
 class formatter<std::basic_string<Char, Traits, Allocator>, Char>
     : public formatter<basic_string_view<Char>, Char> {};
 
+// Fixes issue #4036
+template <typename Char>
+struct formatter<detail::std_string_view<Char>, Char>
+    : public formatter<basic_string_view<Char>, Char> {
+  auto format(const detail::std_string_view<Char>& value,
+              format_context& ctx) const -> decltype(ctx.out()) {
+    using base = formatter<basic_string_view<Char>, Char>;
+    return base::format(value, ctx);
+  }
+};
+
 template <typename Char, size_t N>
 struct formatter<Char[N], Char> : formatter<basic_string_view<Char>, Char> {};
 

test/format-test.cc

@@ -1650,6 +1650,20 @@ TEST(format_test, format_explicitly_convertible_to_std_string_view) {
   EXPECT_EQ("'foo'",
             fmt::format("{}", explicitly_convertible_to_std_string_view()));
 }
+
+struct convertible_to_std_string_view {
+  operator std::string_view() const noexcept { return "Hi there"; }
+};
+FMT_BEGIN_NAMESPACE
+template <>
+class formatter<convertible_to_std_string_view>
+    : public formatter<std::string_view> {};
+FMT_END_NAMESPACE
+
+TEST(format_test, format_implicitly_convertible_and_inherits_string_view) {
+  static_assert(fmt::is_formattable<convertible_to_std_string_view>{});
+  EXPECT_EQ("Hi there", fmt::format("{}", convertible_to_std_string_view{}));
+}
 #endif
 
 class Answer {};