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

$ go version 1.14.2

Does this issue reproduce with the latest release?

Yes

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

Linux, amd64

$ go env

What did you do?

It seems go build -tags is broken. In the following, I have three simple files, a.go, b.go, main.go
go build builds fine, go build -tags xxxdbg builds fine, but go build -tags -xxxopt fail (CHK redefined)

[ftian@localhost tmp]$ cat *.go
// +build xxxdbg
// -- this is a.go
package main

func CHK(err error) {
	if err != nil {
		panic(err)
	}
}
// +build xxxopt
// -- this is b.go
package main

func CHK(err error) {
}

// -- this is main.go
package main


import (
	"fmt"
)

func main() {
	CHK(fmt.Errorf("This is an error."));
}

What did you expect to see?

go build -tags xxxdbg builds OK.
go build -tags xxxopt builds OK.
go build FAIL, CHK not defined

What did you see instead?

go build -tags xxxdbg builds OK
go build -tags xxxopt fails, CHK redeclared
go build build OK