go: text/template: wrap errors returned by custom template functions

What version of Go are you using (go version)?

go version go1.13 windows/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

set GO111MODULE=on
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\User\AppData\Local\go-build
set GOENV=C:\Users\User\AppData\Roaming\go\env
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=D:\Go
set GOPRIVATE=
set GOPROXY=direct
set GOROOT=C:\Go\go1.13
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Go\go1.13\pkg\tool\windows_amd64
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=E:\Projects\test\go.mod
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\User\AppData\Local\Temp\go-build112356450=/tmp/go-build -gno-record-gcc-switches

What did you do?

I’m trying to get the error returned by my custom template function. The error is a struct implementing the error interface.

Execute returns template.ExecError and despite the fact that it implements Unwrap() error, calling Unwrap returns *errors.errorsString, not my error implementation.

Thus, there is no way to get the original error.

Code:

package main

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

type MyError struct {
	Value int
}

func (e *MyError) Error() string { return "error message" }

func test(val int) (string, error) {
	return "", &MyError{
		Value: val,
	}
}

func main() {
	t := template.Must(template.New("test").Funcs(template.FuncMap{
		"test": test,
	}).Parse("{{ test 10 }}"))

	err := t.Execute(os.Stdout, nil)

	if err != nil {
		var e *MyError
		fmt.Println(errors.As(err, &e)) // <-- false
	}
}

What did you expect to see?

true

What did you see instead?

false

Additional notes

1. Documentation for template.FuncMap states that:

if the second (error) return value evaluates to non-nil during execution, execution terminates and Execute returns that error

1. The same applies for method calls like {{ .Object.Method }}.

2. Both text/template and html/template are affected.