IG: report enum singletons in "Singleton" inspection

plugins/InspectionGadgets/InspectionGadgetsAnalysis/src/com/siyeh/InspectionGadgetsBundle.properties

@@ -916,6 +916,7 @@ constant.declared.in.interface.problem.descriptor=Constant <code>#ref</code> dec
 static.inheritance.problem.descriptor=Interface <code>#ref</code> is implemented only for its 'static' constants #loc
 utility.class.problem.descriptor=Class <code>#ref</code> has only 'static' members, indicating procedural construction #loc
 singleton.problem.descriptor=Class <code>#ref</code> is a singleton #loc
+enum.singleton.problem.descriptor=Enum <code>#ref</code> is a singleton #loc
 final.private.method.problem.descriptor='private' method declared <code>#ref</code> #loc
 noop.method.in.abstract.class.problem.descriptor=No-op Method <code>#ref()</code> should be made abstract #loc
 final.static.method.problem.descriptor='static' method declared <code>#ref</code> #loc

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2003-2010 Dave Griffith, Bas Leijdekkers
+ * Copyright 2003-2017 Dave Griffith, Bas Leijdekkers
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -33,7 +33,9 @@ public class SingletonInspection extends BaseInspection {
   @Override
   @NotNull
   protected String buildErrorString(Object... infos) {
-    return InspectionGadgetsBundle.message("singleton.problem.descriptor");
+    return (Boolean)infos[0] ?
+           InspectionGadgetsBundle.message("enum.singleton.problem.descriptor") :
+           InspectionGadgetsBundle.message("singleton.problem.descriptor");
   }
 
   @Override
@@ -50,11 +52,10 @@ public class SingletonInspection extends BaseInspection {
 
     @Override
     public void visitClass(@NotNull PsiClass aClass) {
-      // no call to super, so that it doesn't drill down to inner classes
       if (!SingletonUtil.isSingleton(aClass)) {
         return;
       }
-      registerClassError(aClass);
+      registerClassError(aClass, aClass.isEnum());
     }
   }
 }

plugins/InspectionGadgets/InspectionGadgetsAnalysis/src/com/siyeh/ig/psiutils/SingletonUtil.java

@@ -33,18 +33,26 @@ public class SingletonUtil {
   private SingletonUtil() {}
 
   public static boolean isSingleton(@Nullable PsiClass aClass) {
-    if (aClass == null || aClass.isInterface() || aClass.isEnum() || aClass.isAnnotationType()) {
+    if (aClass == null ||
+        aClass.isInterface() ||
+        aClass.isAnnotationType() ||
+        aClass instanceof PsiTypeParameter ||
+        aClass instanceof PsiAnonymousClass) {
       return false;
     }
-    if (aClass instanceof PsiTypeParameter || aClass instanceof PsiAnonymousClass) {
-      return false;
+    if (aClass.isEnum()) {
+      if (!ControlFlowUtils.hasChildrenOfTypeCount(aClass, 1, PsiEnumConstant.class)) {
+        return false;
+      }
+      // has at least on accessible instance method
+      return Arrays.stream(aClass.getMethods())
+        .anyMatch(m -> !m.isConstructor() && !m.hasModifierProperty(PsiModifier.PRIVATE) && !m.hasModifierProperty(PsiModifier.STATIC));
     }
-    final PsiMethod[] constructors = getIfOnlyInvisibleConstructors(aClass);
-    if (constructors.length == 0) {
+    if (getIfOnlyInvisibleConstructors(aClass).length == 0) {
       return false;
     }
     final PsiField selfInstance = getIfOneStaticSelfInstance(aClass);
-    return selfInstance != null && newOnlyAssignsToStaticSelfInstance(constructors[0], selfInstance);
+    return selfInstance != null && newOnlyAssignsToStaticSelfInstance(getIfOnlyInvisibleConstructors(aClass)[0], selfInstance);
   }
 
   private static PsiField getIfOneStaticSelfInstance(PsiClass aClass) {

plugins/InspectionGadgets/test/com/siyeh/igtest/classlayout/singleton/Singleton.java

@@ -49,4 +49,21 @@ class <warning descr="Class 'Singleton2' is a singleton">Singleton2</warning> {
     private static class SingletonHolder {
         private static final Singleton2 INSTANCE = new Singleton2();
     }
+}
+enum No1 {
+    ;
+    void f() {}
+}
+enum No2 {
+    ONE, TWO;
+
+    void f() {}
+}
+enum No3 {
+    INSTANCE
+}
+enum <warning descr="Enum 'Yes' is a singleton">Yes</warning> {
+    INSTANCE;
+
+    void f() {}
 }