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cmd/compile: spurious "duplicate case in switch" error #24159

mdempsky opened this issue Feb 27, 2018 · 2 comments


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commented Feb 27, 2018

This code is spec compliant:

package p

type A int
const X A = 0

func f() {
        type A int
        const Y A = 0

        switch interface{}(nil) {
        case X:
        case Y:

go/types and gccgo accept it, but cmd/compile erroneously rejects it with:

a.go:12:2: duplicate case Y (value 0) in switch
        previous case at a.go:4:13

The issue is swt.go:632 is using typeVal to look for value collisions. However, it's using the string representation of types to look for type identity, but this isn't sound as the example above demonstrates.

@mdempsky mdempsky added the NeedsFix label Feb 27, 2018


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commented Feb 27, 2018

The line number for the "previous case" is wrong too. (It's pointing at the constant value declaration, instead of the case clause.)


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commented Mar 3, 2018

Change mentions this issue: cmd/compile: prevent detection of wrong duplicates

@gopherbot gopherbot closed this in b75e8a2 Mar 7, 2018

@golang golang locked and limited conversation to collaborators Mar 7, 2019

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