Produce an error when unmarshaling text protos if a oneof field is set more than once.
PiperOrigin-RevId: 137859856
diff --git a/proto/text_parser_test.go b/proto/text_parser_test.go
index b26500b..d757ab3 100644
--- a/proto/text_parser_test.go
+++ b/proto/text_parser_test.go
@@ -635,6 +635,17 @@
 	if !Equal(m, want) {
 		t.Errorf("\n got %v\nwant %v", m, want)
 	}
+
+	const inOverwrite = `name:"Shrek" number:42`
+	m = new(Communique)
+	testErr := "line 1.13: oneof field 'number' is already set"
+	if err := UnmarshalText(inOverwrite, m); err == nil {
+		t.Errorf("TestOneofParsing: Didn't get expected error: %v", testErr)
+	} else if err.Error() != testErr {
+		t.Errorf("TestOneofParsing: Incorrect error.\nHave: %v\nWant: %v",
+			err.Error(), testErr)
+	}
+
 }
 
 var benchInput string