It's been 15 years since I wrote this so I don't remember exactly, but I think it's just an implementation quirk. See: https://github.com/protocolbuffers/proto

It's been 15 years since I wrote this so I don't remember exactly, but I think it's just an implementation quirk. See: https://github.com/protocolbuffers/protobuf/blob/main/src/google/protobuf/compiler/parser.cc#L1321 https://github.com/protocolbuffers/protobuf/blob/main/src/go... The parser consumes a "-", then consumes a number. The number can be any 64-bit unsigned integer. It is then converted to a double. Finally, if a "-" was seen earlier, it is negated. So by accident, it ends up allowing the range of a 65-bit signed integer.