math/big: wrong answer from Float.Sub

[soniakeys]

Please answer these questions before submitting your issue. Thanks!

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

go version devel +daa0ddd Wed May 24 22:28:33 2017 +0000 linux/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOOS="linux"

What did you do?

Called Float.Sub. Passed arguments of opposite sign and used the second argument for the receiver.

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.
https://play.golang.org/p/NdNWBZDJBv

What did you expect to see?

-5
-5

What did you see instead?

-5
-3

[randall77]

@griesemer

[gopherbot]

CL https://golang.org/cl/44150 mentions this issue.

[ALTree]

I thought this was my fault (cda633b) but apparently not, since I can reproduce this in go1.7

I have sent a (tentative) very small fix here: https://go-review.googlesource.com/c/44150/