8369517: Compilation mismatch for equivalent lambda and method reference

src/jdk.compiler/share/classes/com/sun/tools/javac/code/Source.java

@@ -277,6 +277,7 @@ public enum Feature {
         JAVA_BASE_TRANSITIVE(JDK25, Fragments.FeatureJavaBaseTransitive, DiagKind.PLURAL),
         PRIVATE_MEMBERS_IN_PERMITS_CLAUSE(JDK19),
         ERASE_POLY_SIG_RETURN_TYPE(JDK24),
+        CAPTURE_MREF_RETURN_TYPE(JDK26),
         ;
 
         enum DiagKind {

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Attr.java

@@ -124,6 +124,7 @@ public class Attr extends JCTree.Visitor {
     final ArgumentAttr argumentAttr;
     final MatchBindingsComputer matchBindingsComputer;
     final AttrRecover attrRecover;
+    final boolean captureMRefReturnType;
 
     public static Attr instance(Context context) {
         Attr instance = context.get(attrKey);
@@ -175,6 +176,7 @@ protected Attr(Context context) {
                              Feature.UNCONDITIONAL_PATTERN_IN_INSTANCEOF.allowedInSource(source);
         sourceName = source.name;
         useBeforeDeclarationWarning = options.isSet("useBeforeDeclarationWarning");
+        captureMRefReturnType = Source.Feature.ERASE_POLY_SIG_RETURN_TYPE.allowedInSource(source);
 
         statInfo = new ResultInfo(KindSelector.NIL, Type.noType);
         varAssignmentInfo = new ResultInfo(KindSelector.ASG, Type.noType);
@@ -3832,9 +3834,10 @@ void checkReferenceCompatible(JCMemberReference tree, Type descriptor, Type refT
         }
 
         if (!returnType.hasTag(VOID) && !resType.hasTag(VOID)) {
+            Type capturedResType = captureMRefReturnType ? types.capture(resType) : resType;
             if (resType.isErroneous() ||
-                    new FunctionalReturnContext(checkContext).compatible(resType, returnType,
-                            checkContext.checkWarner(tree, resType, returnType))) {
+                    new FunctionalReturnContext(checkContext).compatible(capturedResType, returnType,
+                            checkContext.checkWarner(tree, capturedResType, returnType))) {
                 incompatibleReturnType = null;
             }
         }

test/langtools/tools/javac/lambda/methodReference/ResultTypeNotBeingCapturedTest.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/**
+ * @test
+ * @bug 8369517
+ * @summary Compilation mismatch for equivalent lambda and method reference
+ * @compile/fail/ref=ResultTypeNotBeingCapturedTest.out -XDrawDiagnostics ResultTypeNotBeingCapturedTest.java
+ */
+
+import java.util.function.Supplier;
+
+class ResultTypeNotBeingCapturedTest {
+    interface X<T> {
+        X<T> self();
+    }
+
+    static X<?> makeX() {return null;}
+
+    static <R> X<R> create(Supplier<? extends R> supplier) {return null;}
+
+    static X<X<?>> methodRef() {
+        return create(ResultTypeNotBeingCapturedTest::makeX).self();
+    }
+
+    static X<X<?>> lambda() {
+        return create(() -> makeX()).self();
+    }
+}

test/langtools/tools/javac/lambda/methodReference/ResultTypeNotBeingCapturedTest.out

@@ -0,0 +1,3 @@
+ResultTypeNotBeingCapturedTest.java:43:66: compiler.err.prob.found.req: (compiler.misc.inconvertible.types: ResultTypeNotBeingCapturedTest.X<ResultTypeNotBeingCapturedTest.X<compiler.misc.type.captureof: 1, ?>>, ResultTypeNotBeingCapturedTest.X<ResultTypeNotBeingCapturedTest.X<?>>)
+ResultTypeNotBeingCapturedTest.java:47:42: compiler.err.prob.found.req: (compiler.misc.inconvertible.types: ResultTypeNotBeingCapturedTest.X<ResultTypeNotBeingCapturedTest.X<compiler.misc.type.captureof: 1, ?>>, ResultTypeNotBeingCapturedTest.X<ResultTypeNotBeingCapturedTest.X<?>>)
+2 errors