spec: definition for iota doesn't make sense for nested ConstDecls

[mdempsky]

This program should print 1, but cmd/compile rejects it with an "undefined: iota" error.

package main

import (
        "fmt"
        "unsafe"
)

const (
        _ = unsafe.Sizeof(func() int {
                const (
                        _ = 1
                        _
                        _
                )
                return 0
        }())

        y = iota
)

func main() {
        fmt.Println(y)
}

[mdempsky]

Two side thoughts:

1. Arguably the Go spec says it should print 3 (not 1, like I say above), because the value of iota is said to reset whenever the const keyword appears in the source, not in the block. I suspect that's just because the Go spec's wording wasn't written with nested const declarations in mind.

2. Should

   const _ = unsafe.Sizeof(func() int {
       var _ int
       return iota
   })
   

   be valid? Arguably iota here is appearing inside a ConstSpec so it should be allowed by the spec, yet cmd/compile, gotype, and gccgo all reject it. (cmd/compile accepts it if you remove the var declaration.)

/cc @griesemer @ianlancetaylor

[griesemer]

Yes, I would agree that there's (among other things) a spec issue here - each scope should have its own iota. I think we can safely make that adjustment in the spec w/o affecting real-life code.

Regarding the comments:

1. Per the strict spec interpretation, it should print 4 rather than 3, no?
2. I think if we fix the spec, the example in 2) should not be permitted.

[mdempsky]

@griesemer Ah, agreed that strict interpretation supports printing 4, not 3. (And so gccgo is actually technically conforming.)

[mdempsky]

Per discussion on #15551, I'm reclassifying this as an issue against the spec. Based on how we resolve this issue, we can then (re)open issues against the compilers / typecheckers to fix their behavior as appropriate.

[gopherbot]

CL https://golang.org/cl/36122 mentions this issue.