spec: add duplicate nil to the implementation restriction for switches

[griesemer]

See #28357 for details.

[mdempsky]

For what it's worth, cmd/compile only warns about duplicate nils for switches on non-interface expressions. That is, it doesn't warn about duplicate nil for switches on interface expressions or at all in map literals.

I'm working on a CL to unify this code. My inclination is to just stop warning about duplicate nils, rather than relax the spec to allow this. We don't even report them correctly, so I doubt anyone's triggering this in practice:

/tmp/r.go:16:2: duplicate case nil (value 0) in switch
	previous case at <unknown line number>

[gopherbot]

Change https://golang.org/cl/152544 mentions this issue: cmd/compile: unify duplicate const detection logic

[mdempsky]

Oh, actually the warnings about duplicate nil in interface switch expressions was removed just recently in eb6c433 as part of fixing #28078. And that warning did give a correct previous line number.

[griesemer]

The implementation now follows exactly the spec: only duplicate integer, floating-point, and string constants are disallowed.

[ianlancetaylor]

Note that the implementation also rejects duplicate boolean constants.

[griesemer]

@ianlancetaylor That is not the case, neither with 1.17, nor 1.18.

[ianlancetaylor]

When I run https://go.dev/play/p/BHQnDSDJK_T I see "./prog.go:6:37: duplicate key false in map literal".