strconv: ParseFloat seems not working correctly with a large exponent

[hkurokawa]

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

$ go version
go version go1.15.4 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/hiroshi/.cache/go-build"
GOENV="/home/hiroshi/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/hiroshi/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/hiroshi/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/hiroshi/Downloads/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/hiroshi/Downloads/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build943543493=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Run the below program (Go Playground: https://play.golang.org/p/BKUGd_uDl9_Q)

fmt.Println(strconv.ParseFloat("0."+strings.Repeat("0", 100000)+"9e100000", 64)) // 0 <nil>
fmt.Println(strconv.ParseFloat("9"+strings.Repeat("0", 100000)+"e-100000", 64))  // 0 <nil>

What did you expect to see?

I assume the first line should print 0.9 <nil> and the second should print 9 <nil>.

What did you see instead?

I see the below output.

0 <nil>
0 <nil>

[seankhliao]

probably to do with https://golang.org/src/strconv/atof.go#L289 and the comment above:

// optional exponent moves decimal point.
// if we read a very large, very long number,
// just be sure to move the decimal point by
// a lot (say, 100000). it doesn't matter if it's
// not the exact number.