math/big: printing float is not respecting RoundingMode

[mpvl]

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

go version go1.7 darwin/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH=""
GORACE=""
GOROOT="/usr/local/go1.7"
GOTOOLDIR="/usr/local/go1.7/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

What did you do?

Rounded a big.Float using formatting and using SetPrec to the same precision and using the same RoundingMode.
https://play.golang.org/p/wqWAnOzf45

What did you expect to see?

I expected formatting of numbers should also respect the rounding mode.

Not sure if this was intentional, but putting it out there.

What did you see instead?

It does not.

[griesemer]

Note that the behavior for big.Floats is the same as for float64 with respect to printing: https://play.golang.org/p/z_Y_hIg9Cq

I'm not saying there's no issue here, but if there is, it's also in strconv. Haven't had time to investigate in more detail.

[mpvl]

Background; I'm adding this ability to x/text repo for number formatting. Right now I have the unfortunate situation that I can't rely on much of the code in core to do number formatting (or formatting in general). I can also show you what I end up with and then see how to retrofit it in core if there is a desire to not have duplicate code.

[griesemer]

The "issue" here is that the rounding mode for formatting is always RoundToNearestEven. This matches the strconv library.

If you need to round than do it before formatting and you get the desired result: https://play.golang.org/p/iSo7VROcPj