spec: allow P(x) with type P unsafe.Pointer

[griesemer]

The following program compiles and runs (both gc and gccgo):

package main
import "unsafe"
type P unsafe.Pointer
func main() {
    var x uintptr
    _ = P(x)
}

The conversion P(x) should not be legal. According to the spec (
http://tip.golang.org/ref/spec#Package_unsafe ):

"Any pointer or value of underlying type uintptr can be converted into a Pointer
and vice versa."

It does not say "into a (unsafe.)Pointer type" - only _the_ (unsafe.)Pointer
type.

It also opens the door for unsafe-ness to escape. With:

package safe
import "unsafe"
type Pointer unsafe.Pointer

One can now write:

package main
import "safe"
func main() {
    var x uintptr
    _ = safe.Pointer(x)
}

[rsc]

Comment 1:

For what it's worth, Ian and I discussed this at some point in the past and
interpreted the spec as meaning the current behavior. (I think one can read
"a Pointer" as "a value of an unsafe.Pointer type".) Gccgo originally
behaved as requested in this bug, but Ian changed it to match 6g. The
reflect package uses this functionality. It would not be hard to remove but
I also wonder if others are using it. To avoid breaking things, I am
inclined to reword the spec to match what the two compilers do.

[rsc]

Comment 2:

Labels changed: added priority-later, removed priority-triage, compilerbug, gccgo.

Status changed to Thinking.

[rsc]

Comment 3:

Status changed to Started.

[rsc]

Comment 4:

This issue was closed by revision 81eb930.

Status changed to Fixed.