Missing Position when Capture errors

[mhr3]

Running this code produces a ParseError (as expected), but the error doesn't have position information:

package main

import (
	"fmt"
	"time"

	"github.com/alecthomas/participle/v2"
	"github.com/alecthomas/participle/v2/lexer"
)

type Expression struct {
	Alias     string     `@Ident "="`
	Timestamp *Timestamp `@Number`
}

type Timestamp int64

func (ts *Timestamp) Capture(values []string) error {
	t, err := time.Parse("2006", values[0])
	if err != nil {
		return err
	}
	*ts = Timestamp(t.Unix())
	return nil
}

var exprLexer = lexer.MustSimple([]lexer.Rule{
	{"Whitespace", `\s+`, nil},
	{"String", `"(\\"|[^"])*"`, nil},
	{"Number", `[-+]?(\d*\.)?\d+`, nil},
	{"Ident", `[a-zA-Z_]\w*`, nil},
	{"Punct", `[-[!@#$%^&*()+_={}\|:;"'<,>.?/]|]`, nil},
})

func main() {
	parser := participle.MustBuild(&Expression{}, participle.Lexer(exprLexer), participle.Unquote(), participle.Elide("Whitespace"))

	expr := &Expression{}
	err := parser.ParseString("", "foo = 03", expr)
	if err != nil {
		parseErr := err.(participle.Error)
		fmt.Println("error at position", parseErr.Position(), parseErr)
		return
	}
	fmt.Println("success:", expr.Alias, "=", *expr.Timestamp)
}

The output is:

error at position 0:0 Expression.Timestamp: parsing time "03" as "2006": cannot parse "03" as "2006"

(using github.com/alecthomas/participle/v2 v2.0.0-alpha7)

[alecthomas]

I did a bit of digging and this is definitely a bug. Unfortunately the fix is non-trivial so it might take a little while.