lambda: do not skip generic method when non-generic method exist in the same class

java/java-psi-api/src/com/intellij/psi/LambdaHighlightingUtil.java

@@ -36,7 +36,7 @@ public class LambdaHighlightingUtil {
   @Nullable
   public static String checkInterfaceFunctional(@NotNull PsiClass psiClass, String interfaceNonFunctionalMessage) {
     if (psiClass instanceof PsiTypeParameter) return null; //should be logged as cyclic inference
-    final List<MethodSignature> signatures = LambdaUtil.findFunctionCandidates(psiClass);
+    final List<HierarchicalMethodSignature> signatures = LambdaUtil.findFunctionCandidates(psiClass);
     if (signatures == null) return interfaceNonFunctionalMessage;
     if (signatures.isEmpty()) return "No target method found";
     if (signatures.size() == 1) {
@@ -113,7 +113,7 @@ public class LambdaHighlightingUtil {
     final PsiClass aClass = resolveResult.getElement();
     if (aClass != null) {
       if (aClass instanceof PsiTypeParameter) return null; //should be logged as cyclic inference
-      final List<MethodSignature> signatures = LambdaUtil.findFunctionCandidates(aClass);
+      final List<HierarchicalMethodSignature> signatures = LambdaUtil.findFunctionCandidates(aClass);
       if (signatures != null && signatures.size() == 1) {
         final MethodSignature functionalMethod = signatures.get(0);
         if (functionalMethod.getTypeParameters().length > 0) return "Target method is generic";

java/java-psi-api/src/com/intellij/psi/LambdaUtil.java

@@ -112,7 +112,7 @@ public class LambdaUtil {
   public static boolean isFunctionalClass(PsiClass aClass) {
     if (aClass != null) {
       if (aClass instanceof PsiTypeParameter) return false;
-      final List<MethodSignature> signatures = findFunctionCandidates(aClass);
+      final List<HierarchicalMethodSignature> signatures = findFunctionCandidates(aClass);
       return signatures != null && signatures.size() == 1;
     }
     return false;
@@ -142,7 +142,7 @@ public class LambdaUtil {
   @Nullable
   static MethodSignature getFunction(PsiClass psiClass) {
     if (psiClass == null) return null;
-    final List<MethodSignature> functions = findFunctionCandidates(psiClass);
+    final List<HierarchicalMethodSignature> functions = findFunctionCandidates(psiClass);
     if (functions != null && functions.size() == 1) {
       return functions.get(0);
     }
@@ -176,13 +176,13 @@ public class LambdaUtil {
   }
 
   @Nullable
-  private static List<MethodSignature> hasSubsignature(List<MethodSignature> signatures) {
-    for (MethodSignature signature : signatures) {
+  private static List<HierarchicalMethodSignature> hasSubsignature(List<HierarchicalMethodSignature> signatures) {
+    for (HierarchicalMethodSignature signature : signatures) {
       boolean subsignature = true;
-      for (MethodSignature methodSignature : signatures) {
+      for (HierarchicalMethodSignature methodSignature : signatures) {
         if (!signature.equals(methodSignature)) {
-          if (!MethodSignatureUtil.isSubsignature(signature, methodSignature) && 
-              methodSignature.getTypeParameters().length == 0) {
+          if (!MethodSignatureUtil.isSubsignature(signature, methodSignature) &&
+              !skipMethod(signature, methodSignature)) {
             subsignature = false;
             break;
           }
@@ -193,10 +193,20 @@ public class LambdaUtil {
     return signatures;
   }
 
+  private static boolean skipMethod(HierarchicalMethodSignature signature,
+                                    HierarchicalMethodSignature methodSignature) {
+    //not generic
+    if (methodSignature.getTypeParameters().length == 0) {
+      return false;
+    }
+    //foreign class
+    return signature.getMethod().getContainingClass() != methodSignature.getMethod().getContainingClass();
+  }
+
   @Nullable
-  public static List<MethodSignature> findFunctionCandidates(PsiClass psiClass) {
+  public static List<HierarchicalMethodSignature> findFunctionCandidates(PsiClass psiClass) {
     if (psiClass != null && psiClass.isInterface() && !psiClass.isAnnotationType()) {
-      final List<MethodSignature> methods = new ArrayList<MethodSignature>();
+      final List<HierarchicalMethodSignature> methods = new ArrayList<HierarchicalMethodSignature>();
       final Collection<HierarchicalMethodSignature> visibleSignatures = psiClass.getVisibleSignatures();
       for (HierarchicalMethodSignature signature : visibleSignatures) {
         final PsiMethod psiMethod = signature.getMethod();
@@ -364,7 +374,7 @@ public class LambdaUtil {
   @Nullable
   private static PsiType extractFunctionalConjunct(PsiIntersectionType type) {
     PsiType conjunct = null;
-    for (PsiType conjunctType : ((PsiIntersectionType)type).getConjuncts()) {
+    for (PsiType conjunctType : type.getConjuncts()) {
       final PsiMethod interfaceMethod = getFunctionalInterfaceMethod(conjunctType);
       if (interfaceMethod != null) {
         if (conjunct != null && !conjunct.equals(conjunctType)) return null;

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/lambda/highlighting/GenericNotGenericInterfaceMethod.java

@@ -14,3 +14,30 @@ class Test {
     C c = ()-> {};
   }
 }
+
+class Test1 {
+
+  interface F {
+    <X> void  m();
+  }
+
+  {
+    F f = this::g;
+  }
+
+  void g() {}
+}
+
+class Test2 {
+
+  interface F {
+    <X> void  m();
+    void a();
+  }
+
+  {
+    F f = <error descr="Multiple non-overriding abstract methods found">() -> g()</error>;
+  }
+
+  void g() {}
+}

plugins/InspectionGadgets/InspectionGadgetsAnalysis/src/com/siyeh/ig/classlayout/InterfaceMayBeAnnotatedFunctionalInspection.java

@@ -75,7 +75,7 @@ public class InterfaceMayBeAnnotatedFunctionalInspection extends BaseInspection
       if (LambdaHighlightingUtil.checkInterfaceFunctional(aClass) != null) {
         return;
       }
-      final List<MethodSignature> candidates = LambdaUtil.findFunctionCandidates(aClass);
+      final List<HierarchicalMethodSignature> candidates = LambdaUtil.findFunctionCandidates(aClass);
       if (candidates == null || candidates.size() != 1) {
         return;
       }