cmd/gofmt not inserting empty line between "package" and "import" when canonical import is used #33413
Description
TL;DR: in case package foo line is having "canonical import" comment added to it (as per https://golang.org/doc/go1.4#canonicalimports), gofmt fails to insert an empty line between "package" and "import" lines. See example below.
What version of Go are you using (go version)?
$ go version go version go1.12.7 linux/amd64
Does this issue reproduce with the latest release?
What operating system and processor architecture are you using (go env)?
go env Output
$ go env GOARCH="amd64" GOBIN="" GOCACHE="/home/kir/.cache/go-build" GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="linux" GOOS="linux" GOPATH="/home/kir/go" GOPROXY="" GORACE="" GOROOT="/snap/go/4098" GOTMPDIR="" GOTOOLDIR="/snap/go/4098/pkg/tool/linux_amd64" GCCGO="gccgo" 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-build408267095=/tmp/go-build -gno-record-gcc-switches"
What did you do?
$ cat << EOF >> a.go
> package foo // import "foo.com/go-foo"
> import "sync"
> var m sync.Mutex
> EOF
$ gofmt -d a.go
diff -u a.go.orig a.go
--- a.go.orig 2019-08-01 10:45:50.092931444 -0700
+++ a.go 2019-08-01 10:45:50.092931444 -0700
@@ -1,3 +1,4 @@
package foo // import "foo.com/go-foo"
import "sync"
+
var m sync.Mutex
Now, if we remove the canonical import statement, it works as expected:
$ sed -i 's| // .*||' a.go
$ cat a.go
package foo
import "sync"
var m sync.Mutex
$ gofmt -d a.go
diff -u a.go.orig a.go
--- a.go.orig 2019-08-01 10:47:12.961603804 -0700
+++ a.go 2019-08-01 10:47:12.961603804 -0700
@@ -1,3 +1,5 @@
package foo
+
import "sync"
+
var m sync.Mutex
What did you expect to see?
package foo
+
import "sync"
+
var m sync.Mutex
What did you see instead?
package foo // import "foo.com/go-foo"
import "sync"
+
var m sync.Mutex