Fix check for nil interface

Author: antonmedv

expr_test.go

@@ -876,12 +876,15 @@ func TestExpr_map_default_values_compile_check(t *testing.T) {
 func TestExpr_calls_with_nil(t *testing.T) {
 	env := map[string]interface{}{
 		"equals": func(a, b interface{}) interface{} {
+			assert.Nil(t, a, "a is not nil")
+			assert.Nil(t, b, "b is not nil")
 			return a == b
 		},
+		"is": is{},
 	}
 
 	p, err := expr.Compile(
-		"a == nil && equals(b, nil)",
+		"a == nil && equals(b, nil) && is.Nil(c)",
 		expr.Env(env),
 		expr.Operator("==", "equals"),
 		expr.AllowUndefinedVariables(),
@@ -1020,3 +1023,9 @@ func (m mockMapStringStringEnv) Split(s, sep string) []string {
 }
 
 type mockMapStringIntEnv map[string]int
+
+type is struct{}
+
+func (is) Nil(a interface{}) bool {
+	return a == nil
+}

vm/vm.go

@@ -251,12 +251,13 @@ func (vm *VM) Run(program *Program, env interface{}) interface{} {
 			in := make([]reflect.Value, call.Size)
 			for i := call.Size - 1; i >= 0; i-- {
 				param := vm.pop()
-				if param == nil {
-					// In case of nil interface{} (nil type) use this hack,
+				if param == nil && reflect.TypeOf(param) == nil {
+					// In case of nil value and nil type use this hack,
 					// otherwise reflect.Call will panic on zero value.
-					param = reflect.ValueOf(&in).Elem()
+					in[i] = reflect.ValueOf(&param).Elem()
+				} else {
+					in[i] = reflect.ValueOf(param)
 				}
-				in[i] = reflect.ValueOf(param)
 			}
 			out := fetchFn(env, call.Name).Call(in)
 			vm.push(out[0].Interface())
@@ -275,12 +276,13 @@ func (vm *VM) Run(program *Program, env interface{}) interface{} {
 			in := make([]reflect.Value, call.Size)
 			for i := call.Size - 1; i >= 0; i-- {
 				param := vm.pop()
-				if param == nil {
-					// In case of nil interface{} (nil type) use this hack,
+				if param == nil && reflect.TypeOf(param) == nil {
+					// In case of nil value and nil type use this hack,
 					// otherwise reflect.Call will panic on zero value.
-					param = reflect.ValueOf(&in).Elem()
+					in[i] = reflect.ValueOf(&param).Elem()
+				} else {
+					in[i] = reflect.ValueOf(param)
 				}
-				in[i] = reflect.ValueOf(param)
 			}
 			out := fetchFn(vm.pop(), call.Name).Call(in)
 			vm.push(out[0].Interface())