8325859: Potential information loss during type inference

Reviewed-by: mcimadamore

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

@@ -206,7 +206,11 @@ public class Infer {
                     //propagate outwards if needed
                     if (shouldPropagate) {
                         //propagate inference context outwards and exit
-                        minContext.dupTo(resultInfo.checkContext.inferenceContext());
+                        InferenceContext duppedTo = resultInfo.checkContext.inferenceContext();
+                        minContext.dupTo(duppedTo);
+                        if (minContext != inferenceContext) {
+                            duppedTo.parentIC = inferenceContext;
+                        }
                         deferredAttrContext.complete();
                         return mt;
                     }

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

@@ -83,6 +83,14 @@ public class InferenceContext {
     Types types;
     Infer infer;
 
+    /* when an inference context (IC) is minimized, the minimized inference context (MIC) contains a
+     * proper subset of IC's inference vars (IC_IV). In other words there will be at least one inference variable T
+     * that belongs to IC_IV which doesn't belong to MIC_IV. We need the field below to, among other things,
+     * check for set membership for cases where the minimized context or any other context derived from it
+     * needs to deal with an inference variable that has been eliminated from IC_IV while minimizing it
+     */
+    InferenceContext parentIC;
+
     public InferenceContext(Infer infer, List<Type> inferencevars) {
         this(infer, inferencevars, inferencevars.map(infer.fromTypeVarFun));
     }
@@ -243,7 +251,17 @@ public class InferenceContext {
      * fully instantiated, it will still be available in the resulting type.
      */
     Type asInstType(Type t) {
-        return types.subst(t, inferencevars, instTypes());
+        ListBuffer<Type> from = new ListBuffer<>();
+        ListBuffer<Type> to = new ListBuffer<>();
+        from.addAll(inferencevars);
+        to.addAll(instTypes());
+        InferenceContext next = parentIC;
+        while (next != null) {
+            from.addAll(next.inferencevars);
+            to.addAll(next.instTypes());
+            next = next.parentIC;
+        }
+        return types.subst(t, from.toList(), to.toList());
     }
 
     List<Type> asInstTypes(List<Type> ts) {
@@ -551,8 +569,12 @@ public class InferenceContext {
 
     @Override
     public String toString() {
-        return "Inference vars: " + inferencevars + '\n' +
-               "Undet vars: " + undetvars;
+        String result = "Inference vars: " + inferencevars + '\n' +
+               "Undet vars: " + undetvars + '\n';
+        if (parentIC != null) {
+            result += "\nParent : " + parentIC.toString();
+        }
+        return result;
     }
 
     /* Method Types.capture() generates a new type every time it's applied

test/langtools/tools/javac/inference_context_min/SupplementaryInferenceContextTest.java

@@ -0,0 +1,47 @@
+/*
+ * 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 8325859
+ * @summary potential information loss during type inference
+ * @modules jdk.compiler/com.sun.tools.javac.util
+ */
+
+import com.sun.tools.javac.util.Assert;
+
+public class SupplementaryInferenceContextTest {
+    static String result;
+    public static void main(String... args) {
+        runT(() -> supplyNull(Integer.valueOf(1)));
+        Assert.check(result.equals("class java.lang.Integer"));
+    }
+
+    static <R, X> R supplyNull(X... varargs) {
+        result = varargs.getClass().getComponentType().toString();
+        System.err.println("result is =" + result);
+        return null;
+    }
+
+    static <T> void runT(Runnable runnable) { runnable.run(); }
+}