Sign extend arguments of smaller types passed to %ll? (#265)

format.cc

@@ -328,11 +328,10 @@ class ArgConverter : public fmt::internal::ArgVisitor<ArgConverter<T>, void> {
     } else {
       if (is_signed) {
         arg_.type = Arg::LONG_LONG;
-        // Convert value to unsigned type before converting to long long for
-        // consistency with glibc's printf even though in general it's UB:
+        // glibc's printf doesn't sign extend arguments of smaller types:
         //   std::printf("%lld", -42);  // prints "4294967254"
-        arg_.long_long_value =
-            static_cast<typename fmt::internal::MakeUnsigned<U>::Type>(value);
+        // but we don't have to do the same because it's a UB.
+        arg_.long_long_value = value;
       } else {
         arg_.type = Arg::ULONG_LONG;
         arg_.ulong_long_value =

test/printf-test.cc

@@ -306,8 +306,8 @@ void TestLength(const char *length_spec, U value) {
   }
   using fmt::internal::MakeUnsigned;
   if (sizeof(U) <= sizeof(int) && sizeof(int) < sizeof(T)) {
-    signed_value = unsigned_value =
-        static_cast<typename MakeUnsigned<unsigned>::Type>(value);
+    signed_value = value;
+    unsigned_value = static_cast<typename MakeUnsigned<unsigned>::Type>(value);
   } else {
     signed_value = static_cast<typename MakeSigned<T>::Type>(value);
     unsigned_value = static_cast<typename MakeUnsigned<T>::Type>(value);