NPE

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

@@ -65,7 +65,10 @@ public abstract class InputOutputConstraintFormula implements ConstraintFormula
             }
           }
 
-          collectReturnTypeVariables(session, psiExpression, substitutor.substitute(interfaceMethod.getReturnType()), result);
+          final PsiType returnType = interfaceMethod.getReturnType();
+          if (returnType != null) {
+            collectReturnTypeVariables(session, psiExpression, substitutor.substitute(returnType), result);
+          }
 
           return result;
         }

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/lambda/incomplete/MissedFunctionalInterfaceMethodReturnType.java

@@ -0,0 +1,22 @@
+class Test {
+
+  interface A {
+    A f();
+  }
+  interface B {}
+
+  static abstract class C implements A, B {}
+  static abstract class D implements A, B {}
+
+  interface I<T> {
+    <error descr="Invalid method declaration; return type required">m</error>(T arg);
+  }
+
+  void bar(C c) {
+    foo(c, x -> x.f());
+    foo(c, x -> x);
+  }
+
+  <T> void foo(T t1, I<T> t3) {}
+
+}

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

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2000-2014 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.intellij.codeInsight.daemon.lambda;
+
+import com.intellij.codeInsight.daemon.LightDaemonAnalyzerTestCase;
+import com.intellij.codeInspection.LocalInspectionTool;
+import com.intellij.codeInspection.unusedSymbol.UnusedSymbolLocalInspection;
+import com.intellij.openapi.projectRoots.Sdk;
+import com.intellij.testFramework.IdeaTestUtil;
+import org.jetbrains.annotations.NonNls;
+import org.jetbrains.annotations.NotNull;
+
+public class FunctionalExpressionIncompleteHighlightingTest extends LightDaemonAnalyzerTestCase {
+  @NonNls static final String BASE_PATH = "/codeInsight/daemonCodeAnalyzer/lambda/incomplete";
+
+  @NotNull
+  @Override
+  protected LocalInspectionTool[] configureLocalInspectionTools() {
+    return new LocalInspectionTool[]{
+      new UnusedSymbolLocalInspection(),
+    };
+  }
+
+  public void testMissedFunctionalInterfaceMethodReturnType() throws Exception {
+    doTest();
+  }
+
+  private void doTest() {
+    doTest(false);
+  }
+
+  private void doTest(final boolean checkWarnings) {
+    doTestNewInference(BASE_PATH + "/" + getTestName(false) + ".java", checkWarnings, false);
+  }
+
+  @Override
+  protected Sdk getProjectJDK() {
+    return IdeaTestUtil.getMockJdk18();
+  }
+}