--- a/src/jdk.compiler/share/classes/com/sun/tools/javac/code/Source.java	Tue Dec 18 07:33:07 2018 -0500
+++ b/src/jdk.compiler/share/classes/com/sun/tools/javac/code/Source.java	Tue Dec 18 16:22:46 2018 -0500
@@ -189,7 +189,8 @@
         SWITCH_MULTIPLE_CASE_LABELS(JDK13, Fragments.FeatureMultipleCaseLabels, DiagKind.PLURAL),
         SWITCH_RULE(JDK13, Fragments.FeatureSwitchRules, DiagKind.PLURAL),
         SWITCH_EXPRESSION(JDK13, Fragments.FeatureSwitchExpressions, DiagKind.PLURAL),
-        RAW_STRING_LITERALS(JDK13, Fragments.FeatureRawStringLiterals, DiagKind.PLURAL);
+        RAW_STRING_LITERALS(JDK13, Fragments.FeatureRawStringLiterals, DiagKind.PLURAL),
+        RETURN_TYPE_SUBSTITUTABLE_SAME_TYPE(JDK13);
 
         enum DiagKind {
             NORMAL,