cmd/vet: check for duplicate cases in switches

[josharian]

Consider:

type A [1]int

func f(x A) {
    switch x {
    case A([1]int{1}):
    case A([1]int{1}):
    }
}

This is almost certainly a typo; the second case is unreachable. The compiler may reject duplicate constant cases in expression switches, but not e.g. nil or composite literals containing constants. (See #15896 for discussion.) Perhaps cmd/vet should step in to help catch these likely bugs. This issue is a reminder to investigate.

[mvdan]

Why not make this a compiler error instead of a vet warning? The compiler already errors on more trivial duplicate case expressions.

[ianlancetaylor]

@mvdan Let's try it in vet first. It's not always clear when two different cases are identical. Do they have to be written precisely the same way? What if one says 1 and one says int(1)? And so forth.

Also, doing it in the compiler would require a spec change, probably an addition to the implementation restriction in https://golang.org/ref/spec#Switch_statements . Which can of course be done but again requires some thought as to when cases are identical.

[mvdan]

@ianlancetaylor that makes sense, thank you.