KT-11666: Implicit nothing is allowed for overridden function / properties

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

@@ -705,7 +705,7 @@ class DeclarationsChecker(
         descriptor.returnType?.let {
             if (declaration.typeReference == null) {
                 val target = declaration.nameIdentifier ?: declaration
-                if (it.isNothing()) {
+                if (it.isNothing() && !declaration.hasModifier(KtTokens.OVERRIDE_KEYWORD)) {
                     trace.report(
                             (if (declaration is KtProperty) IMPLICIT_NOTHING_PROPERTY_TYPE else IMPLICIT_NOTHING_RETURN_TYPE).on(target)
                     )

compiler/testData/diagnostics/tests/implicitNothing.kt

@@ -34,3 +34,17 @@ class Klass {
         <!UNREACHABLE_CODE!>val <!UNUSED_VARIABLE!>x<!> =<!> y
     }
 }
+
+interface Base {
+    val x: Int
+
+    fun foo(): String
+}
+
+class Derived : Base {
+    // Ok for override
+    
+    override val x = null!!
+
+    override fun foo() = null!!
+}

compiler/testData/diagnostics/tests/implicitNothing.txt

@@ -8,6 +8,23 @@ public fun check(): kotlin.Unit
 public fun foo(): kotlin.Nothing
 public fun gav(): kotlin.Any
 
+public interface Base {
+    public abstract val x: kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class Derived : Base {
+    public constructor Derived()
+    public open override /*1*/ val x: kotlin.Nothing
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ fun foo(): kotlin.Nothing
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
 public final class Klass {
     public constructor Klass()
     public final val y: kotlin.Nothing