Add bit_reference-like formatter

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>

include/fmt/std.h

@@ -8,6 +8,7 @@
 #ifndef FMT_STD_H_
 #define FMT_STD_H_
 
+#include <bitset>
 #include <cstdlib>
 #include <exception>
 #include <memory>
@@ -15,6 +16,7 @@
 #include <type_traits>
 #include <typeinfo>
 #include <utility>
+#include <vector>
 
 #include "format.h"
 #include "ostream.h"
@@ -389,6 +391,50 @@ struct formatter<
 #endif
   }
 };
+
+namespace detail {
+
+template <typename T, typename Enable = void>
+struct has_flip : std::false_type {};
+
+template <typename T>
+struct has_flip<T, void_t<decltype(std::declval<T>().flip())>>
+    : std::true_type {};
+
+template <typename T> struct is_bit_reference_like {
+  static constexpr const bool value =
+      std::is_convertible<T, bool>::value &&
+      std::is_nothrow_assignable<T, bool>::value && has_flip<T>::value;
+};
+
+#ifdef _LIBCPP_VERSION
+
+// Workaround for libc++ incompatibility with C++ standard.
+// According to the Standard, `bitset::operator[] const` returns bool.
+template <typename C>
+struct is_bit_reference_like<std::__bit_const_reference<C>> {
+  static constexpr const bool value = true;
+};
+
+#endif
+
+}  // namespace detail
+
+// We can't use std::vector<bool, Allocator>::reference and
+// std::bitset<N>::reference because the compiler can't deduce Allocator and N
+// in partial specialization.
+FMT_EXPORT
+template <typename BitRef, typename Char>
+struct formatter<BitRef, Char,
+                 enable_if_t<detail::is_bit_reference_like<BitRef>::value>>
+    : formatter<bool, Char> {
+  template <typename FormatContext>
+  FMT_CONSTEXPR auto format(const BitRef& v, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    return formatter<bool, Char>::format(v, ctx);
+  }
+};
+
 FMT_END_NAMESPACE
 
 #endif  // FMT_STD_H_