lambda: accept? inference variables as target type for lambda expression during lambda constraint reduction (IDEA-136435)

java/java-psi-impl/src/com/intellij/psi/impl/source/resolve/graphInference/constraints/LambdaExpressionCompatibilityConstraint.java

@@ -22,6 +22,9 @@ public class LambdaExpressionCompatibilityConstraint implements ConstraintFormul
 
   @Override
   public boolean reduce(InferenceSession session, List<ConstraintFormula> constraints) {
+    if (session.getInferenceVariable(myT) != null) {
+      return true;
+    }
     if (!LambdaUtil.isFunctionalType(myT)) {
       return false;
     }

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/lambda/newLambda/NestedLambdaWithInferenceVariableAsTargetType.java

@@ -0,0 +1,11 @@
+import java.util.function.Supplier;
+
+class Test {
+  {
+    Supplier<Runnable> x = foo(() -> () -> {});
+  }
+
+  static <T> Supplier<T> foo(Supplier<T> delegate) {
+    return null;
+  }
+}

java/java-tests/testSrc/com/intellij/codeInsight/daemon/lambda/NewLambdaHighlightingTest.java

@@ -99,6 +99,10 @@ public class NewLambdaHighlightingTest extends LightDaemonAnalyzerTestCase {
     doTest();
   }
 
+  public void testNestedLambdaWithInferenceVariableAsTargetType() throws Exception {
+    doTest();
+  }
+
   public void testIDEA127596() throws Exception {
     doTest();
   }