Properly check if interface is any

Fixes #744

Author: antonmedv

checker/checker.go

@@ -535,6 +535,14 @@ func (v *checker) MemberNode(node *ast.MemberNode) Nature {
 		}
 	}
 
+	// Not found.
+
+	if name, ok := node.Property.(*ast.StringNode); ok {
+		if node.Method {
+			return v.error(node, "type %v has no method %v", base, name.Value)
+		}
+		return v.error(node, "type %v has no field %v", base, name.Value)
+	}
 	return v.error(node, "type %v[%v] is undefined", base, prop)
 }
 

checker/checker_test.go

@@ -1055,7 +1055,7 @@ func TestCheck_builtin_without_call(t *testing.T) {
 		err   string
 	}{
 		{`len + 1`, "invalid operation: + (mismatched types func(...interface {}) (interface {}, error) and int) (1:5)\n | len + 1\n | ....^"},
-		{`string.A`, "type func(interface {}) string[string] is undefined (1:8)\n | string.A\n | .......^"},
+		{`string.A`, "type func(interface {}) string has no field A (1:8)\n | string.A\n | .......^"},
 	}
 
 	for _, test := range tests {

checker/nature/nature.go

@@ -74,7 +74,7 @@ func (n Nature) Elem() Nature {
 func (n Nature) AssignableTo(nt Nature) bool {
 	if n.Nil {
 		// Untyped nil is assignable to any interface, but implements only the empty interface.
-		if nt.Type != nil && nt.Type.Kind() == reflect.Interface {
+		if isAny(nt) {
 			return true
 		}
 	}
@@ -84,6 +84,13 @@ func (n Nature) AssignableTo(nt Nature) bool {
 	return n.Type.AssignableTo(nt.Type)
 }
 
+func (n Nature) NumMethods() int {
+	if n.Type == nil {
+		return 0
+	}
+	return n.Type.NumMethod()
+}
+
 func (n Nature) MethodByName(name string) (Nature, bool) {
 	if n.Type == nil {
 		return unknown, false

checker/nature/utils.go

@@ -6,6 +6,10 @@ import (
 	"github.com/expr-lang/expr/internal/deref"
 )
 
+func isAny(nt Nature) bool {
+	return nt.Kind() == reflect.Interface && nt.NumMethods() == 0
+}
+
 func fieldName(field reflect.StructField) string {
 	if taggedName := field.Tag.Get("expr"); taggedName != "" {
 		return taggedName

checker/types.go

@@ -74,12 +74,16 @@ func isUnknown(nt Nature) bool {
 	switch {
 	case nt.Type == nil && !nt.Nil:
 		return true
-	case nt.Kind() == reflect.Interface:
+	case isAny(nt):
 		return true
 	}
 	return false
 }
 
+func isAny(nt Nature) bool {
+	return nt.Kind() == reflect.Interface && nt.NumMethods() == 0
+}
+
 func isInteger(nt Nature) bool {
 	switch nt.Kind() {
 	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:

test/interface_method/interface_method_test.go renamed to test/interface/interface_method_test.go

@@ -1,4 +1,4 @@
-package interface_method_test
+package interface_test
 
 import (
 	"testing"

test/interface/interface_test.go

@@ -0,0 +1,40 @@
+package interface_test
+
+import (
+	"testing"
+
+	"github.com/expr-lang/expr"
+	"github.com/expr-lang/expr/internal/testify/assert"
+)
+
+type StoreInterface interface {
+	Get(string) int
+}
+
+type StoreImpt struct{}
+
+func (f StoreImpt) Get(s string) int {
+	return 42
+}
+
+func (f StoreImpt) Set(s string, i int) bool {
+	return true
+}
+
+type Env struct {
+	Store StoreInterface `expr:"store"`
+}
+
+func TestInterfaceHide(t *testing.T) {
+	var env Env
+	p, err := expr.Compile(`store.Get("foo")`, expr.Env(env))
+	assert.NoError(t, err)
+
+	out, err := expr.Run(p, Env{Store: StoreImpt{}})
+	assert.NoError(t, err)
+	assert.Equal(t, 42, out)
+
+	_, err = expr.Compile(`store.Set("foo", 100)`, expr.Env(env))
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "type interface_test.StoreInterface has no method Set")
+}