fix(vm): handle non-comparable groupBy keys (#940)

Check that the groupBy predicate result is comparable before
using it as a map key. Previously, a non-comparable type such
as a slice would cause a raw Go runtime panic. Now it produces
a clear error message instead.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

Author: thevilledev

test/fuzz/fuzz_test.go

@@ -65,6 +65,7 @@ func FuzzExpr(f *testing.F) {
 		regexp.MustCompile(`sortBy order argument must be a string`),
 		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`),
 	}
 
 	env := NewEnv()

vm/vm.go

@@ -578,6 +578,9 @@ func (vm *VM) Run(program *Program, env any) (_ any, err error) {
 		case OpGroupBy:
 			scope := vm.currScope
 			key := vm.pop()
+			if key != nil && !reflect.TypeOf(key).Comparable() {
+				panic(fmt.Sprintf("cannot use %T as a key for groupBy: type is not comparable", key))
+			}
 			scope.Acc.(groupBy)[key] = append(scope.Acc.(groupBy)[key], scope.Item())
 
 		case OpSortBy:

vm/vm_test.go

@@ -487,6 +487,11 @@ func TestVM_GroupAndSortOperations(t *testing.T) {
 				"odd":  {1, 3, 5},
 			},
 		},
+		{
+			name:        "group by with non-comparable key",
+			expr:        `groupBy([1, 2, 3], [#, # + 1])`, // predicate returns a slice, which is not comparable
+			expectError: "not comparable",
+		},
 		{
 			name:        "invalid sort order",
 			expr:        `sortBy([1, 2, 3], #, "invalid")`,