Comment 1:

if i use _ = f(), then f() is actually called, so I'd expect
5/(1-v) to be also evaluated (and panic) for consistency.
http://play.golang.org/p/qPMDjks3So

Comment 2:

The spec doesn't say that assignments to _ can be "optimized away". It's an assignment
like any other. It just says that the value, once assigned to a _, cannot be  recovered
from the _. That said, perhaps we should be more explicit about it,
An implementation is free to optimize as much as it wants as long as there's no semantic
change. If an expression panics upon evaluation, it needs to keep panicking after the
optimization.
I believe we've had issues with this in the past (_ = f()). I'd say this is an
implementation bug. Always good to check what gccgo does.

Comment 3:

So division can be skipped, but slicing out of bounds seems to still panic:
http://play.golang.org/p/VISwXHnZQ_
func main() {
    defer func() {
        if e := recover(); e != nil {
            println("Panic")
        }
    }()
    i := 1
    s := make([]byte, i + 1)
    _ = 1 + s[5]
    println("No panic")
}

Comment 4:

Looks like a bug in gc to me:
http://play.golang.org/p/B3T7Wt8GFq
division appears to be ignored as possible source of panics when it comes to the check
whether the rhs can be "optimized away".
Changing to a compiler bug.

Labels changed: added release-none, repo-main.

Owner changed to ---.

Status changed to Accepted.

Does anybody know whether this has been fixed?

/cc @josharian @randall77 @mdempsky

This is fixed in the SSA backend. It is not fixed for the legacy compiler (aka all the other architectures besides amd64).

Doesn't look like it's fixed:

package main

func main() {
    f(0)
}

func f(x int) {
    _ = 1 / x
}

runs w/o complaints.

I don't think there are any. It was fixed by happenstance, not deliberately. We could add a +build amd64 test.

@griesemer that program does panic for me at tip.

@griesemer, yeah... I see it correctly failing at tip:

bradfitz@laptop ~$ GOROOT=$HOME/go1.6 go run div.go
bradfitz@laptop ~$ GOROOT=$HOME/go go run div.go
panic: runtime error: integer divide by zero

goroutine 1 [running]:
panic(0x4b660, 0xc82000a0c0)
        /Users/bradfitz/go/src/runtime/panic.go:500 +0x18e
main.main()
        /Users/bradfitz/div.go:4 +0x14
exit status 2

I sent https://go-review.googlesource.com/21325

CL https://golang.org/cl/21325 mentions this issue.

My apologies. I thought I had an updated build but obviously I didn't (at home).