Fix case-insensitive field matching when both exported and unexported fields exist

copier.go

@@ -875,5 +875,41 @@ func fieldByName(v reflect.Value, name string, caseSensitive bool) reflect.Value
 		return v.FieldByName(name)
 	}
 
-	return v.FieldByNameFunc(func(n string) bool { return strings.EqualFold(n, name) })
+	// For case-insensitive matching, prioritize exported fields
+	// First try exact match (might be the exported field)
+	if field := v.FieldByName(name); field.IsValid() {
+		return field
+	}
+
+	// If exact match fails, perform case-insensitive matching
+	// but prioritize exported fields
+	var candidates []reflect.Value
+	var candidateNames []string
+
+	structType := v.Type()
+	for i := 0; i < structType.NumField(); i++ {
+		fieldType := structType.Field(i)
+		if strings.EqualFold(fieldType.Name, name) {
+			field := v.Field(i)
+			candidates = append(candidates, field)
+			candidateNames = append(candidateNames, fieldType.Name)
+		}
+	}
+
+	// If there are multiple candidate fields, prioritize exported ones
+	for i, candidate := range candidates {
+		fieldName := candidateNames[i]
+		// Check if field is exported (starts with uppercase letter)
+		if len(fieldName) > 0 && unicode.IsUpper(rune(fieldName[0])) {
+			return candidate
+		}
+	}
+
+	// If no exported field found, return the first match (preserve original behavior)
+	if len(candidates) > 0 {
+		return candidates[0]
+	}
+
+	// If no match found, return zero value
+	return reflect.Value{}
 }

copier_case_insensitive_test.go

@@ -0,0 +1,71 @@
+package copier
+
+import (
+	"testing"
+)
+
+// Test case for issue where case-insensitive field matching
+// fails when there are both exported and unexported fields with similar names
+func TestCaseInsensitiveFieldMatching(t *testing.T) {
+	// Simulate protobuf generated struct with both unexported 'state' and exported 'State' fields
+	type ProtoStruct struct {
+		state      uint32
+		PageNumber int32 `json:"pageNumber"`
+		State      int32 `json:"state"`
+	}
+
+	type SourceStruct struct {
+		PageNumber int32 `json:"pageNumber"`
+		State      int32 `json:"state"`
+	}
+
+	source := SourceStruct{
+		PageNumber: 1,
+		State:      99,
+	}
+
+	dest := &ProtoStruct{}
+
+	err := Copy(dest, &source)
+	if err != nil {
+		t.Fatalf("Copy failed: %v", err)
+	}
+
+	// Verify that the exported State field was copied correctly
+	if dest.State != source.State {
+		t.Errorf("State field not copied correctly. Expected: %d, Got: %d", source.State, dest.State)
+	}
+
+	if dest.state == uint32(source.State) {
+		t.Errorf("state field not copied correctly. Expected: %d, Got: %d", source.State, dest.state)
+	}
+
+	if dest.PageNumber != source.PageNumber {
+		t.Errorf("PageNumber field not copied correctly. Expected: %d, Got: %d", source.PageNumber, dest.PageNumber)
+	}
+}
+
+// Test that exact case matching still works
+func TestExactCaseMatching(t *testing.T) {
+	type Source struct {
+		Name string
+		Age  int
+	}
+
+	type Dest struct {
+		Name string
+		Age  int
+	}
+
+	source := Source{Name: "John", Age: 30}
+	dest := &Dest{}
+
+	err := Copy(dest, &source)
+	if err != nil {
+		t.Fatalf("Copy failed: %v", err)
+	}
+
+	if dest.Name != source.Name || dest.Age != source.Age {
+		t.Errorf("Fields not copied correctly. Expected: %+v, Got: %+v", source, *dest)
+	}
+}