strconv: FormatFloat wrong rounding

[vendelin8]

Go version

go version go1.21.6 linux/amd64

Output of go env in your module/workspace:

GO111MODULE='auto'
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/panda/.cache/go-build'
GOENV='/home/panda/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/panda/og/pkg/mod'
GOOS='linux'
GOPATH='/home/panda/og'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/lib/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.21.6'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD=''
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build3295958002=/tmp/go-build -gno-record-gcc-switches'

What did you do?

https://go.dev/play/p/RyJdylUB8cm

What did you see happen?

the result is 3.50

What did you expect to see?

the result should be 3.51 based on math rounding rules

[dominikh]

3.505 cannot be represented exactly in a binary floating point number. The actual value is 3.50499999999999989341858963598497211933135986328125.