env GO111MODULE=off

! go fmt does-not-exist

go fmt -n exclude
stdout 'exclude[/\\]x\.go'
stdout 'exclude[/\\]x_linux\.go'

-- exclude/empty/x.txt --
-- exclude/ignore/_x.go --
package x
-- exclude/x.go --
// +build linux,!linux

package x
-- exclude/x_linux.go --
// +build windows

package x