What does 'go version' print?

go version 1.2.1 windows/amd64

What steps reproduce the problem?
Run this minimal program:

package main

import "fmt"

func main() {
    length := (1 << 63) - 1
    a := make([]struct{}, length/(1<<32))
    b := make([]struct{}, length/(1<<32))
    a = append(a, b...)
    fmt.Println(len(a), cap(a))
}

Note that this *cannot* be reproduced on the Playground. Even if you substitute the
value of length with the 32-bit MaxInt (since the Playground seems to use a size of 4
instead of 8 for ints), you'll correctly get a panic from this operation.

What happened?

It will print -2 7638104968020361214 (that is, the slice has len -2 and cap
7638104968020361214).

What should have happened instead?

Probably a panic of some sort.

Please provide any additional information below.

This seems to only happen with slices of type struct{}. I tried to reproduce the error
with a []int, and it always panicked in some way. One other odd node, if instead of doing

a = append(a,b...)

you do

a = append(a,struct{}{})

It will correctly panic with the error:

panic: runtime error:  runtime: allocation size out of range

One confusing thing is that the panic the Playground gives for these operations is:

panic: runtime error: growslice: cap out of range

So that may be an additional bug, or may be expected.