somewhat OT but: ended up in the 'go/constant' package's source from there somehow and there saw the comment on "Prevent external implementations" [1] regarding using a non-exported function in the definition of 'constant.Value' interface.

Here is an external implementation of constant.Value:

https://play.golang.org/p/3dVFCT2-kN8

[1]: https://golang.org/src/go/constant/value.go?#L58