Added formatter for bit_reference-like types (#3570)

* Add test for std::vector<bool>::reference

Co-authored-by: Felix <felix-antoine.constantin@polymtl.ca>
Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>

* Add test for std::bitset<N>::reference

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

* Add test for const std::bitset<N>::reference and const std::vector<bool>::reference

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

* Add bit_reference-like formatter

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

* Use std::addressof

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

---------

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>
Co-authored-by: Felix <felix-antoine.constantin@polymtl.ca>

include/fmt/core.h

@@ -13,6 +13,7 @@
 #include <cstring>  // std::strlen
 #include <iterator>
 #include <limits>
+#include <memory>  // std::addressof
 #include <string>
 #include <type_traits>
 
@@ -1281,9 +1282,9 @@ template <typename Context> class value {
   FMT_INLINE value(const named_arg_info<char_type>* args, size_t size)
       : named_args{args, size} {}
 
-  template <typename T> FMT_CONSTEXPR FMT_INLINE value(T& val) {
+  template <typename T> FMT_CONSTEXPR20 FMT_INLINE value(T& val) {
     using value_type = remove_const_t<T>;
-    custom.value = const_cast<value_type*>(&val);
+    custom.value = const_cast<value_type*>(std::addressof(val));
     // Get the formatter type through the context to allow different contexts
     // have different extension points, e.g. `formatter<T>` for `format` and
     // `printf_formatter<T>` for `printf`.

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_

test/std-test.cc

@@ -7,6 +7,7 @@
 
 #include "fmt/std.h"
 
+#include <bitset>
 #include <stdexcept>
 #include <string>
 #include <vector>
@@ -221,3 +222,17 @@ TEST(std_test, exception) {
   }
 #endif
 }
+
+TEST(std_test, format_bit_reference) {
+  std::bitset<2> bs(1);
+  EXPECT_EQ(fmt::format("{} {}", bs[0], bs[1]), "true false");
+  std::vector<bool> v = {true, false};
+  EXPECT_EQ(fmt::format("{} {}", v[0], v[1]), "true false");
+}
+
+TEST(std_test, format_const_bit_reference) {
+  const std::bitset<2> bs(1);
+  EXPECT_EQ(fmt::format("{} {}", bs[0], bs[1]), "true false");
+  const std::vector<bool> v = {true, false};
+  EXPECT_EQ(fmt::format("{} {}", v[0], v[1]), "true false");
+}