Add support for _BitInt on clang

Issue #4007

Author: Arghnews

include/fmt/base.h

@@ -427,6 +427,14 @@ enum class uint128_opt {};
 template <typename T> auto convert_for_visit(T) -> monostate { return {}; }
 #endif
 
+#ifndef FMT_USE_BITINT
+#  if FMT_CLANG_VERSION && FMT_CLANG_VERSION >= 1400
+#    define FMT_USE_BITINT 1
+#  else
+#    define FMT_USE_BITINT 0
+#  endif
+#endif
+
 // Casts a nonnegative integer to unsigned.
 template <typename Int>
 FMT_CONSTEXPR auto to_unsigned(Int value) -> make_unsigned_t<Int> {
@@ -1484,6 +1492,20 @@ template <typename Context> struct arg_mapper {
   FMT_MAP_API auto map(double val) -> double { return val; }
   FMT_MAP_API auto map(long double val) -> long double { return val; }
 
+#if FMT_USE_BITINT
+#  pragma clang diagnostic push
+#  pragma clang diagnostic ignored "-Wbit-int-extension"
+  template <class T, int N = 0> struct is_bitint : std::false_type {};
+  template <int N> struct is_bitint<_BitInt(N)> : std::true_type {};
+  template <int N> struct is_bitint<unsigned _BitInt(N)> : std::true_type {};
+
+  template <class T, FMT_ENABLE_IF(is_bitint<remove_cvref_t<T>>::value)>
+  FMT_MAP_API auto map(T&& val) -> decltype(val) {
+    return val;
+  }
+#  pragma clang diagnostic pop
+#endif
+
   FMT_MAP_API auto map(char_type* val) -> const char_type* { return val; }
   FMT_MAP_API auto map(const char_type* val) -> const char_type* { return val; }
   template <typename T, typename Char = char_t<T>,

test/format-test.cc

@@ -2442,3 +2442,30 @@ FMT_END_NAMESPACE
 TEST(format_test, ustring) {
   EXPECT_EQ(fmt::format("{}", ustring()), "ustring");
 }
+
+#if FMT_USE_BITINT
+#  pragma clang diagnostic push
+#  pragma clang diagnostic ignored "-Wbit-int-extension"
+using BitIntS7 = _BitInt(7);
+using BitIntU32 = unsigned _BitInt(32);
+FMT_BEGIN_NAMESPACE
+template <> struct formatter<BitIntS7> : formatter<int8_t> {};
+
+template <> struct formatter<BitIntU32> : formatter<uint32_t> {};
+FMT_END_NAMESPACE
+
+TEST(format_test, bitint) {
+  EXPECT_EQ(fmt::format("{}", BitIntS7{0}), "0");
+  EXPECT_EQ(fmt::format("{}", BitIntU32{4294967295}), "4294967295");
+
+  auto a = BitIntS7{0};
+  auto b = BitIntU32{4294967295};
+  const auto c = BitIntS7{0};
+  const auto d = BitIntU32{4294967295};
+  EXPECT_EQ(fmt::format("{}", a), "0");
+  EXPECT_EQ(fmt::format("{}", b), "4294967295");
+  EXPECT_EQ(fmt::format("{}", c), "0");
+  EXPECT_EQ(fmt::format("{}", d), "4294967295");
+}
+#  pragma clang diagnostic pop
+#endif