Prohibit inner sealed classes

#KT-16233 Fixed

compiler/frontend/src/org/jetbrains/kotlin/resolve/ModifiersChecker.kt

@@ -169,6 +169,13 @@ object ModifierCheckerCore {
         result += compatibilityForClassesRegister(PRIVATE_KEYWORD, ABSTRACT_KEYWORD)
 
         result += incompatibilityRegister(CROSSINLINE_KEYWORD, NOINLINE_KEYWORD)
+
+        // 1. subclasses contained inside a sealed class can not be instantiated, because their constructors needs
+        // an instance of an outer sealed (effectively abstract) class
+        // 2. subclasses of a non-top-level sealed class must be declared inside the class
+        // (see the KEEP https://github.com/Kotlin/KEEP/blob/master/proposals/sealed-class-inheritance.md)
+        result += incompatibilityRegister(SEALED_KEYWORD, INNER_KEYWORD)
+
         return result
     }
 

compiler/testData/diagnostics/tests/modifiers/IllegalModifiers.kt

@@ -156,3 +156,7 @@ class IllegalModifiers10
 <!INCOMPATIBLE_MODIFIERS!>const<!> constructor()
 
 class IllegalModifiers11 <!INCOMPATIBLE_MODIFIERS!>private<!> <!INCOMPATIBLE_MODIFIERS!>protected<!> constructor()
+
+class Outer {
+    <!INCOMPATIBLE_MODIFIERS!>inner<!> <!INCOMPATIBLE_MODIFIERS!>sealed<!> class Inner
+}

compiler/testData/diagnostics/tests/modifiers/IllegalModifiers.txt

@@ -132,6 +132,20 @@ package illegal_modifiers {
         public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
     }
 
+    public final class Outer {
+        public constructor Outer()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+        public sealed inner class Inner {
+            private constructor Inner()
+            public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+            public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+            public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+        }
+    }
+
     public final interface T {
         public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
         public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int