fix(vm): handle nil arguments in variadic funcs (#868)

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 <ville@vesilehto.fi>
Co-authored-by: Anton Medvedev <anton@medv.io>

Author: thevilledev

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)
 				}