fix: make std::bitset formattable again

It used to be formattable via operator<<(ostream&) implicitly. Make it
formattable again, but this time via formatter specialization.

include/fmt/std.h

@@ -145,6 +145,32 @@ FMT_END_NAMESPACE
 #endif
 
 FMT_BEGIN_NAMESPACE
+FMT_EXPORT
+template <std::size_t N, typename Char>
+struct formatter<std::bitset<N>, Char> : nested_formatter<string_view> {
+ private:
+  // Functor because C++11 doesn't support generic lambdas.
+  struct writer {
+    const std::bitset<N>& bs;
+
+    template <typename OutputIt>
+    FMT_CONSTEXPR OutputIt operator()(OutputIt out) {
+      for (auto pos = N; pos > 0; --pos) {
+        out = detail::write<Char>(out, bs[pos - 1] ? Char('1') : Char('0'));
+      }
+
+      return out;
+    }
+  };
+
+ public:
+  template <typename FormatContext>
+  auto format(const std::bitset<N>& bs, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    return write_padded(ctx, writer{bs});
+  }
+};
+
 FMT_EXPORT
 template <typename Char>
 struct formatter<std::thread::id, Char> : basic_ostream_formatter<Char> {};

test/std-test.cc

@@ -237,6 +237,14 @@ TEST(std_test, format_const_bit_reference) {
   EXPECT_EQ(fmt::format("{} {}", v[0], v[1]), "true false");
 }
 
+TEST(std_test, format_bitset) {
+  const std::bitset<6> bs(42);
+  EXPECT_EQ(fmt::format("{}", bs), "101010");
+  EXPECT_EQ(fmt::format("{:.4}", bs), "101010");
+  EXPECT_EQ(fmt::format("{:0>8}", bs), "00101010");
+  EXPECT_EQ(fmt::format("{:-^12}", bs), "---101010---");
+}
+
 TEST(std_test, format_atomic) {
   std::atomic<bool> b(false);
   EXPECT_EQ(fmt::format("{}", b), "false");