cmd/compile: finite scaling of complex infinity gives complex NaN

[kortschak]

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

$ go version
go version go1.14.2 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="on"
GOARCH="amd64"
GOBIN="/home/user/bin"
GOCACHE="/home/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/user"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/user/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/user/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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-build634029336=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Run the code at https://play.golang.org/p/0WqB9HZZFwA

What did you expect to see?

should be complex inf: (+Inf+Infi)

What did you see instead?

should be complex inf: (NaN+NaNi)

[bmkessler]

This is working as intended. The issue is the untyped constant 42 is converted to a complex number with zero imaginary part and the multiplication between Inf and 0.0 is NaN.

cmplx.Inf()*42 == complex(math.Inf(1), math.Inf(1)) * (42 + 0i)
... == (math.Inf(1)*42-math.Inf(1)*0) + (math.Inf(1)*0+math.Inf(1)*42) i
... == (math.Inf(1) - math.NaN()) + (math.NaN() + math.Inf()) i
... == math.Nan() +math.NaN() i

To scale by a real number rather than multiply by a complex number with zero imaginary part you need to write this as:

x := cmplx.Inf()
x = complex(42 * real(x), 42 * imag(x))

See #38780 for further discussion.

[kortschak]

Yeah, OK. The spec is terse on this. As mentioned in the linked issue, perhaps that could be improved.