spec: clarify that make does not panic for negative capacity hint

[nobishino]

What version of Go are you using (go version)?

The spec for Go1.18

https://go.dev/ref/spec (Version of March 10, 2022)

Does this issue reproduce with the latest release?

Yes.

What did you do?

I read the following sentence from the spec, and run the following code. (Go playground link is here: https://go.dev/play/p/whw_4ZBzsx_K)

If n is negative or larger than m at run time, a run-time panic occurs.

func main() {
	n := -1
	m := make(map[int]int, n) // does not panic
	fmt.Println(m)
}

What did you expect to see?

I expect the program to panic, because n is negative and the sentence does not exclude the case make is used for creating a map.

What did you see instead?

It did not panic.

I think the spec should clarify that make(map[int]int, n) for negative n does not (necessarily) cause run-time panic.

[seankhliao]

cc @golang/runtime

[mknyszek]

Given that a negative n does not panic now, I don't think it's possible to make it panic without breaking a whole bunch of valid Go programs. It kind of makes sense for a map, because it's really just an optimization hint. If the hint doesn't make sense, it's always safe to ignore it.

However, I don't think you're wrong about the spec, it does suggest what you say. I'm not sure what the right answer is here.

CC @ianlancetaylor and @griesemer

[randall77]

If n is negative or larger than m at run time, a run-time panic occurs.

I think this is sentence is intended to describe making slices, not maps. For maps, there is no m. Although I agree it could be clearer.

See #46909 #24308. The consensus there seems to be that with maps, there is a reasonable way to proceed when n is negative, so we don't panic. (Unlike making slices or channels, where there's no reasonable way to continue the program.)

[gopherbot]

Change https://go.dev/cl/411919 mentions this issue: spec: clarify behavior of map size hint for make built-in