Sealed sub-class -> object: handle equals, suggest "add equals"

Before this commit, sealed sub-class without state was considered
a style issue.
After this commit, sealed sub-class without state AND custom equals
is considered a probable bug,
because comparison of its instances is very fragile.
Alternative fix (generate equals & hashCode by identity) is added.

idea/testData/inspectionsLocal/convertSealedSubClassToObject/baseEquals.kt

@@ -0,0 +1,18 @@
+// PROBLEM: none
+
+abstract class Base {
+    open val prop: Int
+        get() = 13
+
+    override fun equals(other: Any?): Boolean {
+        if (other !is Base) return false
+        return prop == other.prop
+    }
+}
+
+sealed class SC : Base() {
+    <caret>class U : SC()
+
+    override val prop: Int
+        get() = 42
+}

idea/testData/inspectionsLocal/convertSealedSubClassToObject/convertSubClassWithModifiers.kt

@@ -1,3 +1,5 @@
+// FIX: Convert sealed sub-class to object
+
 sealed class Sealed
 
 private <caret>class SubSealed : Sealed()

idea/testData/inspectionsLocal/convertSealedSubClassToObject/convertSubClassWithModifiers.kt.after

@@ -1,3 +1,5 @@
+// FIX: Convert sealed sub-class to object
+
 sealed class Sealed
 
 private object SubSealed : Sealed()

idea/testData/inspectionsLocal/convertSealedSubClassToObject/convertSubClassWithParentheses.kt

@@ -1,3 +1,5 @@
+// FIX: Convert sealed sub-class to object
+
 sealed class Sealed
 
 <caret>class SubSealed() : Sealed()

idea/testData/inspectionsLocal/convertSealedSubClassToObject/convertSubClassWithParentheses.kt.after

@@ -1,3 +1,5 @@
+// FIX: Convert sealed sub-class to object
+
 sealed class Sealed
 
 object SubSealed : Sealed()

idea/testData/inspectionsLocal/convertSealedSubClassToObject/convertSubClassWithoutParentheses.kt

@@ -1,3 +1,5 @@
+// FIX: Convert sealed sub-class to object
+
 sealed class Sealed
 
 <caret>class SubSealed : Sealed()

idea/testData/inspectionsLocal/convertSealedSubClassToObject/convertSubClassWithoutParentheses.kt.after

@@ -1,3 +1,5 @@
+// FIX: Convert sealed sub-class to object
+
 sealed class Sealed
 
 object SubSealed : Sealed()

idea/testData/inspectionsLocal/convertSealedSubClassToObject/fakeState.kt

@@ -1,3 +1,4 @@
+// FIX: Convert sealed sub-class to object
 // WITH_RUNTIME
 
 abstract class Base {

idea/testData/inspectionsLocal/convertSealedSubClassToObject/fakeState.kt.after

@@ -1,3 +1,4 @@
+// FIX: Convert sealed sub-class to object
 // WITH_RUNTIME
 
 abstract class Base {

idea/testData/inspectionsLocal/convertSealedSubClassToObject/noEquals.kt

@@ -0,0 +1,13 @@
+// FIX: Generate equals & hashCode by identity
+
+abstract class Base {
+    open val prop: Int
+        get() = 13
+}
+
+sealed class SC : Base() {
+    <caret>class U : SC()
+
+    override val prop: Int
+        get() = 42
+}

idea/testData/inspectionsLocal/convertSealedSubClassToObject/noEquals.kt.after

@@ -0,0 +1,21 @@
+// FIX: Generate equals & hashCode by identity
+
+abstract class Base {
+    open val prop: Int
+        get() = 13
+}
+
+sealed class SC : Base() {
+    class U : SC() {
+        override fun equals(other: Any?): Boolean {
+            return this === other
+        }
+
+        override fun hashCode(): Int {
+            return System.identityHashCode(this)
+        }
+    }
+
+    override val prop: Int
+        get() = 42
+}

idea/testData/inspectionsLocal/convertSealedSubClassToObject/ownEquals.kt

@@ -0,0 +1,9 @@
+// PROBLEM: none
+
+sealed class SC {
+    <caret>class U : SC() {
+        override fun equals(other: Any?): Boolean {
+            return this === other
+        }
+    }
+}

idea/testData/inspectionsLocal/convertSealedSubClassToObject/sealedEquals.kt

@@ -0,0 +1,12 @@
+// PROBLEM: none
+
+sealed class SC {
+    <caret>class U : SC()
+
+    fun foo() = 42
+
+    override fun equals(other: Any?): Boolean {
+        if (other !is SC) return false
+        return foo() == other.foo()
+    }
+}