cmd/go: go refuses to compile hex floats on +build go1.13 file

[kstenerud]

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

$ go version
go version go1.13.10 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/karl/.cache/go-build"
GOENV="/home/karl/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/karl/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/karl/tmp/floattest/go.mod"
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-build659173498=/tmp/go-build -gno-record-gcc-switches"

What did you do?

go.mod:

module floattest

go 1.11

main.go:

package floattest

func returnFloat(v float64) float64 {
	return v
}

main_1_13_test.go:

// +build go1.13

package floattest

import (
	"testing"
)

func TestStuff(t *testing.T) {
	expected := 0x1.5fp20
	actual := returnFloat(expected)
	if actual != expected {
		t.Errorf("Expected %v but got %v\n", expected, actual)
	}
}

command: go test

What did you expect to see?

PASS
ok  	floattest	0.002s

What did you see instead?

# floattest [floattest.test]
./main_1_13_test.go:10:14: hexadecimal floating-point literals requires go1.13 or later (-lang was set to go1.11; check go.mod)
FAIL	floattest [build failed]

Since the build settings will not compile this file on go < 1.13, this shouldn't be an error.

[cuonglm]

But you use go.1.13.10 to run go test, so the file is built.

[kstenerud]

Yes, so there should be no error, since go 1.13 supports hex float notation.

[OneOfOne]

You specified go 1.11 in go.mod, so I'd say that's expected, it'd actually be a bug if it worked.

[OneOfOne]

https://play.golang.org/p/yi6zb76npL_j vs https://play.golang.org/p/Lh5tpyOIyx7

[kstenerud]

So how to I make my project buildable under 1.11? I've put the 1.13 stuff in a +build guarded file so that it only builds the 1.13 stuff when building with go 1.13.

[cuonglm]

So how to I make my project buildable under 1.11? I've put the 1.13 stuff in a +build guarded file so that it only builds the 1.13 stuff when building with go 1.13.

Use go1.11.x or 1.12.x to build.

Or maybe drop support for go < 1.13, set go1.13 in go.mod

[kstenerud]

It builds fine under 1.11 and 1.12. It only fails when I try to build using 1.13.

Dropping support for 1.11 and 1.12 seems a silly thing to do when it builds fine under those versions. This is a problem with two compiler rules fighting each other, not a rational thing.

[randall77]

I think this should work. Having a go1.11 mark in the mod file means we shouldn't define a go1.13 build tag.
The bad code never makes it to the compiler. Something earlier in the toolchain is barfing on the input.

@bcmills

[cuonglm]

It builds fine under 1.11 and 1.12. It only fails when I try to build using 1.13.

Dropping support for 1.11 and 1.12 seems a silly thing to do when it builds fine under those versions. This is a problem with two compiler rules fighting each other, not a rational thing.

I don't think it's silly.

Go is backward compatible, and also go1.11 and 1.12 are not supported anymore (see release policy)

[cuonglm]

@randall77 How about passing the current go version to -lang if the version is greater than the one specific in go.mod?

[kortschak]

@kstenerud Doesn't doing something like this work for you? https://play.golang.org/p/nqejHFvguRv

[kstenerud]

-- go.mod --
module play.ground

go 1.13

@kortschak Change this to 1.11 and the problem returns.

[kortschak]

So don't change it to go1.11? When you have go1.11 in the Go version specifier you are saying you want to use Go 1.11 features, but hex floats are a Go 1.13 feature.

[kstenerud]

Then that defeats the whole purpose. If go.mod specifies 1.13, then you can't build for <1.13, and foo_old.go is completely pointless since // +build !go1.13 will never be built.

The point is that I want to have my software package buildable in 1.11 and 1.12, but also use hex floats for more specific floating point test code when building in 1.13+.

[kstenerud]

More specifically: this code doesn't take the current +build settings into account to determine that the currently compiling file (with // +build go1.13) is restricted to 1.13 and up, but rather just fails based on the -lang flag.