This is a panic in the go2go type checker. The current type checker reports an error (undefined: some). I don't think it's worth updating the go2go type checker to better handle erroneous cases, but leaving for @griesemer .
@rvolosatovs This is working as expected on tip (types2, and compiler with -G=3 option). It doesn't produce any error message with go/types at the moment, and that seems like a bug. What you are seeing is likely due to running a version that is not fully up-to-date. The go2go playground will be updated eventually, but currently is a bit behind.