spec: unclear whether error should be reported for non-comparable switch expression in empty switch

[griesemer]

For:

var a struct{ _ func() }
switch a {
}

the compiler reports an error:

cannot switch on a (type struct { _ func() }) (struct containing func() cannot be compared)

Since there's no case in the switch statement, this might be considered ok. The spec is not explicitly stating that this is invalid.

See also #43110.

[gopherbot]

Change https://golang.org/cl/278132 mentions this issue: [dev.typeparams] cmd/compile/internal/types2: report error for invalid (but empty) expr switch

[griesemer]

cc: @ianlancetaylor for input on what gccgo does.

[ianlancetaylor]

gccgo says

foo7.go:5:16: error: cannot switch on value whose type that may not be compared
    5 |         switch a {
      |                ^

So the message needs a little work but it has the same behavior.

[gopherbot]

Change https://golang.org/cl/278452 mentions this issue: compiler: correct grammar in error message