whichOneofField method
Implementation
OneofWellKnownTypes_OneofField whichOneofField() =>
_OneofWellKnownTypes_OneofFieldByTag[$_whichOneof(0)]!;
OneofWellKnownTypes_OneofField whichOneofField() =>
_OneofWellKnownTypes_OneofFieldByTag[$_whichOneof(0)]!;