func computeFieldSize(field: int; value: Opt; ProtoType: type ProtobufExt;
                      skipDefault: static bool): int {....raises: [], gcsafe.}

func isExtension(T: type Protobuf; FieldType: type Opt): bool {....raises: [],
    gcsafe.}

proc readFieldInto(stream: InputStream; value: var Opt; header: FieldHeader;
                   ProtoType: type ProtobufExt): bool {.
    ...raises: [SerializationError, IOError], raises: [], gcsafe.}

func supportsPacked(_: type Opt; ProtoType: type ProtobufExt): bool {.
    ...raises: [], gcsafe.}

proc writeField(stream: OutputStream; field: int; value: Opt;
                ProtoType: type ProtobufExt; skipDefault: static bool = false) {.
    ...raises: [IOError], raises: [], gcsafe.}

template flatType[U](T: type Protobuf; value: Opt[U]): type {..}