`find`, `findIndex`, `first` and `get` are type-checked as the element type where they return nil

[DRMacIver]

first of an empty []int, get with an out-of-range index and find/findIndex with no match are all type-checked as int, so AsInt() with WarnOnAny() accepts them at compile time and Run then fails with invalid operation: int(<nil>); the same find over a []any array is refused at compile time:

package main

import (
	"fmt"

	"github.com/expr-lang/expr"
)

func main() {
	env := map[string]any{
		"ints":  []int{1, 2, 3},
		"anys":  []any{1, 2, 3},
		"empty": []int{},
	}
	for _, src := range []string{
		"find(anys, # > 5)",
		"find(ints, # > 5)",
		"findIndex(ints, # > 5)",
		"first(empty)",
		"get(ints, 9)",
	} {
		program, err := expr.Compile(src, expr.Env(env), expr.AsInt(), expr.WarnOnAny())
		if err != nil {
			fmt.Printf("%-23s compile error: %v\n", src, err)
			continue
		}
		_, err = expr.Run(program, env)
		fmt.Printf("%-23s compiled as %v; Run: %v\n", src, program.Node().Type(), err)
	}
}

Output:

find(anys, # > 5)       compile error: expected int, but got interface {}
find(ints, # > 5)       compiled as int; Run: invalid operation: int(<nil>) (1:1)
 | find(ints, # > 5)
 | ^
findIndex(ints, # > 5)  compiled as int; Run: invalid operation: int(<nil>) (1:1)
 | findIndex(ints, # > 5)
 | ^
first(empty)            compiled as int; Run: invalid operation: int(<nil>) (1:1)
 | first(empty)
 | ^
get(ints, 9)            compiled as int; Run: invalid operation: int(<nil>) (1:1)
 | get(ints, 9)
 | ^

first and get are documented to return nil for an empty array and an out-of-range index (language-definition.md), and WarnOnAny() is documented to make the type checker return an error when the return type is any (configuration.md). With a []int array the static type of these calls is int, so the check passes and the nil only shows up when the program runs. I would expect these to be caught at compile time the way the []any case is, or to have a static type that admits nil.

Tested on expr v1.17.8 and on current master (4b31df3), Go 1.27.1.

BTW, this was found by an automated program that writes property-based tests for various open source projects using hegel (but it has been reviewed by hand before reporting). We've also potentially found (but not yet hand validated) 3 other bugs in expr. You can see the tests at https://github.com/hegeldev/hegel-zoo/tree/main/targets/go/expr. Let us know if you would like us to file the other bugs found and/or contribute the tests. NB the tests are currently LLM generated and probably not yet suitable for inclusion as is, but we're happy to help get them into a better state if you want them.