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cmd/vet: check for duplicate cases in switches #15906

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josharian opened this issue May 31, 2016 · 3 comments
Open

cmd/vet: check for duplicate cases in switches #15906

josharian opened this issue May 31, 2016 · 3 comments
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Analysis Issues related to static analysis (vet, x/tools/go/analysis)
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Unplanned

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@josharian
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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.
@josharian josharian added this to the Unplanned milestone May 31, 2016
@josharian josharian self-assigned this May 31, 2016
@josharian josharian mentioned this issue May 31, 2016
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@mvdan
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mvdan commented Nov 7, 2017

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

@ianlancetaylor
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@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
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mvdan commented Nov 7, 2017

@ianlancetaylor that makes sense, thank you.

@dominikh dominikh mentioned this issue Feb 4, 2020
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@adonovan adonovan added the Analysis Issues related to static analysis (vet, x/tools/go/analysis) label Apr 23, 2023
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@josharian @ianlancetaylor @mvdan @adonovan