reject non-comparable keys in map index and in operator

test/fuzz/fuzz_test.go

@@ -66,6 +66,7 @@ func FuzzExpr(f *testing.F) {
 		regexp.MustCompile(`invalid order .*, expected asc or desc`),
 		regexp.MustCompile(`unknown order, use asc or desc`),
 		regexp.MustCompile(`cannot use .* as a key for groupBy: type is not comparable`),
+		regexp.MustCompile(`cannot use .* as a map key: type is not comparable`),
 	}
 
 	env := NewEnv()

vm/runtime/runtime.go

@@ -63,7 +63,11 @@ func Fetch(from, i any) any {
 		if i == nil {
 			value = v.MapIndex(reflect.Zero(v.Type().Key()))
 		} else {
-			value = v.MapIndex(reflect.ValueOf(i))
+			key := reflect.ValueOf(i)
+			if !key.Type().Comparable() {
+				panic(fmt.Sprintf("cannot use %s as a map key: type is not comparable", key.Type()))
+			}
+			value = v.MapIndex(key)
 		}
 		if value.IsValid() {
 			return value.Interface()
@@ -233,7 +237,11 @@ func In(needle any, array any) bool {
 		if needle == nil {
 			value = v.MapIndex(reflect.Zero(v.Type().Key()))
 		} else {
-			value = v.MapIndex(reflect.ValueOf(needle))
+			key := reflect.ValueOf(needle)
+			if !key.Type().Comparable() {
+				panic(fmt.Sprintf("cannot use %s as a map key: type is not comparable", key.Type()))
+			}
+			value = v.MapIndex(key)
 		}
 		if value.IsValid() {
 			return true

vm/vm_test.go

@@ -518,6 +518,23 @@ func TestVM_GroupAndSortOperations(t *testing.T) {
 	}
 }
 
+// TestVM_NonComparableMapKey checks that indexing a map or using the in
+// operator with a non-comparable key returns a clean error instead of a raw
+// "hash of unhashable type" runtime panic.
+func TestVM_NonComparableMapKey(t *testing.T) {
+	env := map[string]any{"list": []map[string]any{{"a": 1}}}
+	for _, code := range []string{
+		`groupBy(list, 0)[reduce(list, B"")]`,
+		`B"" in groupBy(list, 0)`,
+	} {
+		program, err := expr.Compile(code, expr.Env(env))
+		require.NoError(t, err)
+		_, err = vm.Run(program, env)
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "not comparable")
+	}
+}
+
 // TestVM_SortBy_NonStringOrder tests that sortBy with non-string order
 // returns a proper error instead of panicking (regression test for OSS-Fuzz #477658245).
 func TestVM_SortBy_NonStringOrder(t *testing.T) {