html/template: Unable to execute a method on non-pointer struct where method receiver is a pointer (can't evaluate field error)

[John]

[non-pointer struct].<Method with pointer receiver> cannot be invoked directly in a template.

package main

import (

"fmt"

"bytes"

"html/template"

)

type Wrapper struct {

raw []int

}

func (w *Wrapper) IsZero() bool {

if  len(w.raw) == 0{

return true

}

return false

}

var tmplText = `

{{.IsZero}}

` 

var tmpl = template.Must(template.New("test").Parse(tmplText))

func main() {

wrapped := Wrapper{}

fmt.Println(wrapped.IsZero())

buff := &bytes.Buffer{}

if err := tmpl.Execute(buff, wrapped); err != nil {

panic(err)

}

fmt.Println(buff.String())

}

Will produce: 

panic: template: test:2:3: executing "test" at <.IsZero>: can't evaluate field IsZero in type main.Wrapper

goroutine 1 [running]:
main.main()
	/tmp/sandbox009718456/prog.go:33 +0x200

However, changing wrapped from Wrapper to a *Wrapper will work.  

https://play.golang.org/p/lviUVF770c2

Again this might be expected, but I found it a surprised (because I can clearly call the method outside the template).  In my real life case, this was layers deep in a structs I had to unpack and experiment with to find the cause.

The error wasn't clear as to why this was happening, I could not find documentation within template to hint at a limitation (maybe I missed it or I did not interpret the meaning of something correctly).

What I'm looking from this post:

• Are my assumptions correct or is the proof above invalid
• If the proof is valid, is this expected behavior?
• If expected, is there documentation for this?
• and, can we see a way to have better error output, I know this is probably difficult due to trying to reflect on what is a field vs a method when something can't be found

Also, trying to document the error here so it gets picked up in Google when looking for this error condition, as I was unable to find anything similar.