The assignments

   _ = nil
   var _ = nil

are accepted by gccgo w/o error. However they should not be permitted.

The spec is unclear, but gc does not permit it. Also, there shouldn't be a difference
between

   var a = nil

(not permitted) and the same assignment using _ instead of a. Furthermore:

   _ = 1<<63

is not permitted (integer constant overflow), while

   _ = 1<<62

is ok - indicating that such assignments are treated like other assignments with
initialization expressions. nil should not be a special case.