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 +145,26 @@ FMT_END_NAMESPACE
 #endif
 
 FMT_BEGIN_NAMESPACE
+FMT_EXPORT
+template <std::size_t N, typename Char> struct formatter<std::bitset<N>, Char> {
+  template <typename ParseContext>
+  FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
+    return ctx.begin();
+  }
+
+  template <typename FormatContext>
+  auto format(const std::bitset<N>& bs, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    auto out = ctx.out();
+
+    for (auto pos = N; pos; --pos) {
+      out = detail::write<Char>(out, bs[pos - 1] ? Char('1') : Char('0'));
+    }
+
+    return out;
+  }
+};
+
 FMT_EXPORT
 template <typename Char>
 struct formatter<std::thread::id, Char> : basic_ostream_formatter<Char> {};

test/std-test.cc

@@ -237,6 +237,11 @@ 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");
+}
+
 TEST(std_test, format_atomic) {
   std::atomic<bool> b(false);
   EXPECT_EQ(fmt::format("{}", b), "false");