Description
The encoding/gob package emits an erroneous error message when attempting to encode a map containing irreflexive keys (such as math.NaN()):
https://play.golang.org/p/QgRVchmloT6
The error message it returns is gob: encodeReflectValue: nil element, but encodeReflectValue is not part of the exported or documented API, and there are no nil elements involved.
Ideally, encoding/gob should correctly handle maps with irreflexive keys. That would likely require either a fix for #11104, or a change to map lookups to use representation equivalence instead of == (per #20660 (comment)).
If it isn't feasible for gob to handle irreflexive map keys, that restriction should be documented in the gob package, and the errors that gob methods return should describe the correct problem.
This issue parallels #14427.