fix(vm): handle nil arguments in variadic funcs

Update OpCall to properly handle nil values passed to variadic
functions. Previously, passing nil as the last argument caused
an index out of range panic, and a single nil argument was
incorrectly treated as a nil slice. Added regression tests.

Signed-off-by: Ville Vesilehto <[email protected]>

test/issues/817/issue_test.go

@@ -0,0 +1,33 @@
+package issue_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/expr-lang/expr"
+	"github.com/expr-lang/expr/internal/testify/require"
+)
+
+func TestIssue817_1(t *testing.T) {
+	out, err := expr.Eval(
+		`sprintf("result: %v %v", 1, nil)`,
+		map[string]any{
+			"sprintf": fmt.Sprintf,
+		},
+	)
+	require.NoError(t, err)
+	require.Equal(t, "result: 1 <nil>", out)
+}
+
+func TestIssue817_2(t *testing.T) {
+	out, err := expr.Eval(
+		`thing(nil)`,
+		map[string]any{
+			"thing": func(arg ...any) string {
+				return fmt.Sprintf("result: (%T) %v", arg[0], arg[0])
+			},
+		},
+	)
+	require.NoError(t, err)
+	require.Equal(t, "result: (<nil>) <nil>", out)
+}

vm/vm.go

@@ -330,13 +330,29 @@ func (vm *VM) Run(program *Program, env any) (_ any, err error) {
 			vm.push(runtime.Slice(node, from, to))
 
 		case OpCall:
-			fn := reflect.ValueOf(vm.pop())
+			v := vm.pop()
+			if v == nil {
+				panic("invalid operation: cannot call nil")
+			}
+			fn := reflect.ValueOf(v)
+			if fn.Kind() != reflect.Func {
+				panic(fmt.Sprintf("invalid operation: cannot call non-function of type %T", v))
+			}
+			fnType := fn.Type()
 			size := arg
 			in := make([]reflect.Value, size)
+			isVariadic := fnType.IsVariadic()
+			numIn := fnType.NumIn()
 			for i := int(size) - 1; i >= 0; i-- {
 				param := vm.pop()
 				if param == nil {
-					in[i] = reflect.Zero(fn.Type().In(i))
+					var inType reflect.Type
+					if isVariadic && i >= numIn-1 {
+						inType = fnType.In(numIn - 1).Elem()
+					} else {
+						inType = fnType.In(i)
+					}
+					in[i] = reflect.Zero(inType)
 				} else {
 					in[i] = reflect.ValueOf(param)
 				}