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

[mdempsky]

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]

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

[gopherbot]

Change https://golang.org/cl/98476 mentions this issue: cmd/compile: prevent detection of wrong duplicates