`interface conversion: interface {} is int, not string` when trying to index slice type

[blotus]

Hello,

When trying to upgrade expr from 1.9 to the latest version in crowdsec, some of our automated tests failed with the following error:

WARN[09-03-2023 00:17:41] runtime error : interface conversion: interface {} is int, not string (1:27)
 | "192.168.9.212" in results[0].Overflow.GetSources()
 | ..........................^

After some debugging, I managed to narrow down the issue and reproduce it with the following code:

package main

import (
	"fmt"

	"github.com/antonmedv/expr"
)

type EventsSlice []Event

func (b EventsSlice) Len() int {
	return 42
}

type Event struct {
	Foo string
}

func main() {
	env := map[string]interface{}{
		"results": EventsSlice{{"bar"}},
	}

	code := `results[0].Foo`

	program, err := expr.Compile(code, expr.Env(env))
	if err != nil {
		fmt.Printf("error compiling expression: %s\n", err)
		return
	}

	fmt.Printf("%s", program.Disassemble())

	output, err := expr.Run(program, env)
	if err != nil {
		fmt.Printf("error running expression: %s\n", err)
		return
	}

	fmt.Println(output)
}

and gives this output:

0	OpLoadFast	0	results
1	OpPush	1	0
2	OpFetch
3	OpFetchField	2	{Foo [0]}
error running expression: interface conversion: interface {} is int, not string (1:8)
 | results[0].Foo
 | .......^

It seems that if a slice type has a non-pointer receiver, you cannot index it (if you change the Len() definition to be a pointer receiver, expr has no issue using the variable).

This issue is probably related to #271, which you did not manage to reproduce at the time.

Go version: go version go1.20.1 linux/amd64

[antonmedv]

Thanks for the fix! Released v1.12.2 with a fix.