findLast/findLastIndex crash inside map literal expressions

[aura-sohil]

Description

findLast() and findLastIndex() produce a runtime error when used as values inside { } map literal expressions. All other predicate-based builtin functions (find, findIndex, filter, all, any, count, none, one) work correctly in the same position.

Minimal Reproduction

package main

import (
	"fmt"
	"github.com/expr-lang/expr"
)

func main() {
	// These work fine:
	run(`findLast([1.0, 2.0, 3.0, 4.0, 5.0], # > 3)`)        // OK: 5
	run(`{"r": find([1.0, 2.0, 3.0, 4.0, 5.0], # > 3)}`)      // OK: map[r:4]
	run(`{"r": findIndex([1.0, 2.0, 3.0, 4.0, 5.0], # > 3)}`)  // OK: map[r:3]
	run(`{"r": filter([1.0, 2.0, 3.0, 4.0, 5.0], # > 3)}`)     // OK: map[r:[4 5]]
	run(`{"r": all([1.0, 2.0, 3.0, 4.0, 5.0], # > 0)}`)        // OK: map[r:true]

	// These crash:
	run(`{"r": findLast([1.0, 2.0, 3.0, 4.0, 5.0], # > 3)}`)      // ERROR
	run(`{"r": findLastIndex([1.0, 2.0, 3.0, 4.0, 5.0], # > 3)}`)  // ERROR
}

func run(expression string) {
	program, err := expr.Compile(expression)
	if err != nil {
		fmt.Printf("COMPILE ERROR: %s\n  %v\n", expression, err)
		return
	}
	result, err := expr.Run(program, nil)
	if err != nil {
		fmt.Printf("RUN ERROR: %s\n  %v\n", expression, err)
		return
	}
	fmt.Printf("OK: %s\n  %v\n", expression, result)
}

Error

RUN ERROR: {"r": findLast([1.0, 2.0, 3.0, 4.0, 5.0], # > 3)}
  interface conversion: interface {} is bool, not string (1:1)
   | {"r": findLast([1.0, 2.0, 3.0, 4.0, 5.0], # > 3)}
   | ^

Versions Affected

Tested on v1.17.6 and v1.17.8 — both reproduce. The bug appears to be long-standing.

Analysis

The compilation succeeds — the error is purely at runtime (expr.Run). The predicate # > 3 returns a bool, and somewhere in the VM execution path for findLast/findLastIndex inside a MapNode, a value is type-asserted as string when it is actually bool.

find and findIndex (which iterate forward) work fine in the same position. Only the reverse-iterating variants (findLast, findLastIndex) are affected.

Workaround

Extract findLast/findLastIndex into a let binding before the map literal:

let r = findLast([1.0, 2.0, 3.0, 4.0, 5.0], # > 3); {"result": r}