ponies: Compiler doesn't warn on statically knowable bounds errors.

[gopherbot]

by alexjray.ncsu:

Before filing a bug, please check whether it has been fixed since
the latest release: run "hg pull -u" and retry what you did to
reproduce the problem.  Thanks.

What steps will reproduce the problem?
1. v := make([]string,4)
2. v[5] = "foo"
3. compile & run

What is the expected output? What do you see instead?
Compiler issue some kind of warning. Nothing (compiles fine).
When run, generates a stacktrace.

What is your $GOOS? linux $GOARCH? 386


Which revision are you sync'ed to?  (hg log -l 1)
changeset:   4030:52ad1606d994
tag:         tip
user:        Rob Pike <r@golang.org>
date:        Fri Nov 13 13:15:18 2009 -0800
summary:     Explain why


Please provide any additional information below.

Attachments:

1. foo.go (229 bytes)

[agl]

Comment 1:

Status changed to Accepted.

[robpike]

Comment 2:

Status changed to LongTerm.

[rsc]

Comment 3:

Labels changed: added compilerbug.

[rsc]

Comment 4:

Labels changed: added priority-later.

[rsc]

Comment 6:

Labels changed: added priority-someday, removed priority-later.

[rsc]

Comment 7:

Labels changed: added repo-main.

[rsc]

Comment 8:

Adding Release=None to all Priority=Someday bugs.

Labels changed: added release-none.

[rsc]

Moving priority-someday to the Unplanned milestone.

[odeke-em]

Should we change the title of this bug to categorize it under "cmd/compile: ". What does "Ponies: " mean?
/cc @josharian @bradfitz

[mdempsky]

"Ponies" is how we used to label long-term hopeful feature requests.

This seems like a feature we could probably implement in the compiler today using SSA. I think that would go against our "no warnings" policy though.

[odeke-em]

Thanks for the explanation @mdempsky.

[josharian]

Yes, I think we know enough to detect this case at compile time now. But as we don't have compiler warnings, and refusing the compile the code would be wrong, the only thing we could do here is compile this code more efficiently. Since it's just going to panic anyway, and no one will ever write this code with intent to actually use it, there's no point.

If there's anywhere this belongs, it is vet. With help from SSA this should be detectable. I wonder how common it is. In any case, that should be a separate issue (feel free to open one, anyone). Closing this one.