fix #844: disallow direct access to unexported struct fields

Author: diegommm

checker/checker_test.go

@@ -760,8 +760,8 @@ func TestCheck_TaggedFieldName(t *testing.T) {
 
 	config := conf.CreateNew()
 	expr.Env(struct {
-		x struct {
-			y bool `expr:"bar"`
+		X struct {
+			Y bool `expr:"bar"`
 		} `expr:"foo"`
 	}{})(config)
 	expr.AsBool()(config)

checker/nature/utils.go

@@ -52,12 +52,13 @@ func (s *structData) structField(c *Cache, parentEmbed *structData, name string)
 	// Lookup own fields first.
 	for ; s.ownIdx < s.numField; s.ownIdx++ {
 		field := s.rType.Field(s.ownIdx)
-		// BUG: we should skip if !field.IsExported() here
-
 		if field.Anonymous && s.anonIdx < 0 {
 			// start iterating anon fields on the first instead of zero
 			s.anonIdx = s.ownIdx
 		}
+		if !field.IsExported() {
+			continue
+		}
 		fName, ok := fieldName(field.Name, field.Tag)
 		if !ok || fName == "" {
 			// name can still be empty for a type created at runtime with