Fix handling of implicit conversion to integral types larger than int (#507)

test/format-test.cc

@@ -1234,6 +1234,16 @@ TEST(FormatterTest, FormatIntLocale) {
   EXPECT_EQ("1--234--567", format("{:n}", 1234567));
 }
 
+struct ConvertibleToLongLong {
+  operator fmt::LongLong() const {
+    return fmt::LongLong(1) << 32;
+  }
+};
+
+TEST(FormatterTest, FormatConvertibleToLongLong) {
+  EXPECT_EQ("100000000", format("{:x}", ConvertibleToLongLong()));
+}
+
 TEST(FormatterTest, FormatFloat) {
   EXPECT_EQ("392.500000", format("{0:f}", 392.5f));
 }