Report warnings on overrides with wrong types nullability

^KT-48899 Fixed

core/compiler.common/src/org/jetbrains/kotlin/types/AbstractTypeChecker.kt

@@ -445,9 +445,10 @@ object AbstractTypeChecker {
         return true
     }
 
+    @OptIn(ObsoleteTypeKind::class)
     private fun TypeSystemContext.isCommonDenotableType(type: KotlinTypeMarker): Boolean =
         type.typeConstructor().isDenotable() &&
-                !type.isDynamic() && !type.isDefinitelyNotNullType() &&
+                !type.isDynamic() && !type.isDefinitelyNotNullType() && !type.isNotNullTypeVariable() &&
                 type.lowerBoundIfFlexible().typeConstructor() == type.upperBoundIfFlexible().typeConstructor()
 
     fun effectiveVariance(declared: TypeVariance, useSite: TypeVariance): TypeVariance? {
@@ -702,7 +703,8 @@ object AbstractNullabilityChecker {
             if (superType.isMarkedNullable()) return true
 
             // i.e. subType is definitely not null
-            if (subType.isDefinitelyNotNullType()) return true
+            @OptIn(ObsoleteTypeKind::class)
+            if (subType.isDefinitelyNotNullType() || subType.isNotNullTypeVariable()) return true
 
             // i.e. subType is captured type, projection of which is marked not-null
             if (subType is CapturedTypeMarker && subType.isProjectionNotNull()) return true

core/compiler.common/src/org/jetbrains/kotlin/types/model/TypeSystemContext.kt

@@ -377,6 +377,11 @@ interface TypeSystemContext : TypeSystemOptimizationContext {
 
     fun KotlinTypeMarker.isDefinitelyNotNullType(): Boolean = asSimpleType()?.asDefinitelyNotNullType() != null
 
+    // This kind of types is obsolete (expected to be removed at 1.7) and shouldn't be used further in a new code
+    // Now, such types are being replaced with definitely non-nullable types
+    @ObsoleteTypeKind
+    fun KotlinTypeMarker.isNotNullTypeVariable(): Boolean = false
+
     fun KotlinTypeMarker.hasFlexibleNullability() =
         lowerBoundIfFlexible().isMarkedNullable() != upperBoundIfFlexible().isMarkedNullable()
 
@@ -495,3 +500,6 @@ fun requireOrDescribe(condition: Boolean, value: Any?) {
         "Unexpected: value = '$value'$typeInfo"
     }
 }
+
+@RequiresOptIn("This kinds of type is obsolete and should not be used until you really need it")
+annotation class ObsoleteTypeKind