package main

import (
    "fmt"
    "html/template"
    "io/ioutil"
    "log"
    "path"
    "strings"
)

type A struct {
    I []string
}

func main() {
    for i, tpl := range tpls {
        t, err := template.New("foo").Funcs(funcs).Parse(tpl)

        if err != nil {
            log.Fatal(err)
        }

        a := A{I: []string{"abc"}}

        err = t.Execute(ioutil.Discard, a)

        if err != nil {
            fmt.Printf("%d failed: %s\n", i, err)
        }
    }
}

var tpls = []string{
    // Without surrounding div executes OK
    `{{ range .I }}{{ if in . "a" }}{{ join . "a.jpg" }}{{ else  }}a.jpg{{ end }}{{ end }}`,
    // This also works fine
    `<div style="background: url({{ range .I }}{{ if in . "a" }}ab.jpg{{ else  }}a.jpg{{ end }}{{ end }})">FOO</div>`,
    // This fails
    `<div style="background: url({{ range .I }}{{ if in . "a" }}{{ join . "a.jpg" }}{{ else  }}a.jpg{{ end }}{{ end }})">FOO</div>`,
}
var funcs = map[string]interface{}{
    "join": func(a, b string) string { return path.Join(a, b) },
    "in":   func(a, b string) bool { return strings.Contains(a, b) },
}