text/template: does not recover from panics where it should

[remyoudompheng]

What steps will reproduce the problem?
1. Compile and run the following program:

package main

import (
  "errors"
  "fmt"
  "os"
  "text/template"
)

const tpl = `
An error has occurred in {{ .Context }}! Message: {{ .Error.Error }}
`

type Message struct {
  Context string
  Error error
}

func main() {
  t := template.New("T")
  t = template.Must(t.Parse(tpl))

  err := Message{"context", errors.New("some error")}
  errT := t.Execute(os.Stderr, err)
  if errT != nil {
      fmt.Fprintf(os.Stderr, "\ninternal template error: %s", errT)
  }

  err.Error = nil
  errT = t.Execute(os.Stderr, err)
  if errT != nil {
      fmt.Fprintf(os.Stderr, "\ninternal template error: %s", errT)
  }
}


What is the expected output?

An error has occurred in context! Message: some error

An error has occurred in context! Message: 
internal template error: Call of method on nil interface value

What do you see instead?

An error has occurred in context! Message: some error

An error has occurred in context! Message: panic: Call of method on nil interface value
[recovered]
    panic: interface conversion: string is not error: missing method Error

goroutine 1 [running]:
----- stack segment boundary -----
text/template.errRecover(0x7ff5ec9bbee0, 0x7ff5ec9bb100)
    /opt/remy/go/src/pkg/text/template/exec.go:84 +0xaf
----- stack segment boundary -----
reflect.Value.call(0x490650, 0xf84000d450, 0x13a, 0x4b7f54, 0x6c6c614300000004, ...)
    /opt/remy/go/src/pkg/reflect/value.go:367 +0x1f8
reflect.Value.Call(0x490650, 0xf84000d450, 0x13a, 0x52f1a8, 0x0, ...)
    /opt/remy/go/src/pkg/reflect/value.go:330 +0x85
text/template.(*state).evalCall(0xf840019480, 0x4a0130, 0xf84000d440, 0x192, 0x490650,
...)
    /opt/remy/go/src/pkg/text/template/exec.go:489 +0x677
text/template.(*state).evalField(0xf840019480, 0x4a0130, 0xf84000d440, 0x192, 0x4d92d9,
...)
    /opt/remy/go/src/pkg/text/template/exec.go:408 +0x232
text/template.(*state).evalFieldChain(0xf840019480, 0x4a0130, 0xf84000d440, 0x192,
0x490650, ...)
    /opt/remy/go/src/pkg/text/template/exec.go:381 +0x194
text/template.(*state).evalFieldNode(0xf840019480, 0x4a0130, 0xf84000d440, 0x192,
0xf84000d0c0, ...)
    /opt/remy/go/src/pkg/text/template/exec.go:360 +0xa7
text/template.(*state).evalCommand(0xf840019480, 0x4a0130, 0xf84000d440, 0x192,
0xf84000d0e0, ...)
    /opt/remy/go/src/pkg/text/template/exec.go:312 +0x284
text/template.(*state).evalPipeline(0xf840019480, 0x4a0130, 0xf84000d440, 0x192,
0xf8400192d0, ...)
    /opt/remy/go/src/pkg/text/template/exec.go:290 +0xfd
text/template.(*state).walk(0xf840019480, 0x4a0130, 0xf84000d440, 0x192, 0xf8400192a0,
...)
    /opt/remy/go/src/pkg/text/template/exec.go:124 +0xde
text/template.(*state).walk(0xf840019480, 0x4a0130, 0xf84000d440, 0x192, 0xf8400193c0,
...)
    /opt/remy/go/src/pkg/text/template/exec.go:133 +0x5f9
text/template.(*Template).Execute(0xf84001e080, 0xf840019300, 0xf84001a010, 0x4a0120,
0xf84000d440, ...)
    /opt/remy/go/src/pkg/text/template/exec.go:112 +0x1d3
main.main()
    /tmp/main.go:30 +0x2fa

Which compiler are you using (5g, 6g, 8g, gccgo)?

6g

Which operating system are you using?

linux

Which revision are you using?  (hg identify)

52ae6fbcc97a+ tip

Please provide any additional information below.

[remyoudompheng]

Comment 1:

I suggest the following change as a fix: http://golang.org/cl/5527046/

[robpike]

Comment 2:

This issue was closed by revision f5d024a.

Status changed to Fixed.