This is a bug with both the existing and the new parser.

Slightly simpler case:

package p

func f() int {
    switch 0 {
    case 0:
        return (func() int)(nil)()
        fallthrough
    default:
    }
    return 0
}

CL https://golang.org/cl/22921 mentions this issue.

Simpler reproducer for posterity in case the patch I mailed won't be merged

package p

func f() int {
    var a int
    switch a {
    case 0:
        return func() int { return 1 }()
        fallthrough
    default:
    }
    return 0
}

anything that creates an autotmp_ in the return line goes. gc compiles

case 0:
        return func() int { return 1 }()
        fallthrough

as

autotmp_0 = (func literal)(); return autotmp_0; fallthrough; <node VARKILL>

The problem is that the casebody function in swt.go, which replaces Op in OXFALL nodes (not-yet-processed fallthrough) with OFALL (processed fallthrough), assumes that the node with the fallthrough statement will always be the last one. In this case, this is false: the last node is the OVARKILL one. For this reason the OXFALL node won't be marked as OFALL, and the fallthrough on the second line ends up being reported as spurious.