Fix: correctly check error type of func return types

expr_test.go

@@ -1646,6 +1646,19 @@ func TestFastCall(t *testing.T) {
 	assert.Equal(t, float64(8), out)
 }
 
+func TestRun_custom_func_returns_an_error_as_second_arg(t *testing.T) {
+	env := map[string]interface{}{
+		"semver": func(value string, cmp string) (bool, error) { return true, nil },
+	}
+
+	p, err := expr.Compile(`semver("1.2.3", "= 1.2.3")`, expr.Env(env))
+	assert.NoError(t, err)
+
+	out, err := expr.Run(p, env)
+	assert.NoError(t, err)
+	assert.Equal(t, true, out)
+}
+
 // Mock types
 
 type mockEnv struct {

vm/vm.go

@@ -12,9 +12,8 @@ import (
 	"github.com/antonmedv/expr/vm/runtime"
 )
 
-var (
-	MemoryBudget int = 1e6
-)
+var MemoryBudget int = 1e6
+var errorType = reflect.TypeOf((*error)(nil)).Elem()
 
 func Run(program *Program, env interface{}) (interface{}, error) {
 	if program == nil {
@@ -302,7 +301,7 @@ func (vm *VM) Run(program *Program, env interface{}) (out interface{}, err error
 				}
 			}
 			out := fn.Call(in)
-			if len(out) == 2 && !runtime.IsNil(out[1]) {
+			if len(out) == 2 && out[1].Type() == errorType && !out[1].IsNil() {
 				panic(out[1].Interface().(error))
 			}
 			vm.push(out[0].Interface())