[Commonizer] ReturnTypeCommonizer: Only allow covariant nullability for covariant types

The return type of any property with a setter can be considered invariant
and should enable covariant nullability commonization

^KT-48567

native/commonizer/src/org/jetbrains/kotlin/commonizer/core/AbstractFunctionOrPropertyCommonizer.kt

@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.commonizer.core
 
 import org.jetbrains.kotlin.commonizer.cir.CirFunctionOrProperty
 import org.jetbrains.kotlin.commonizer.cir.CirName
+import org.jetbrains.kotlin.commonizer.cir.CirProperty
 import org.jetbrains.kotlin.commonizer.cir.CirType
 import org.jetbrains.kotlin.commonizer.core.TypeCommonizer.Options.Companion.default
 import org.jetbrains.kotlin.commonizer.mergedtree.CirKnownClassifiers
@@ -47,7 +48,8 @@ private class ReturnTypeCommonizer(
     override fun invoke(values: List<CirFunctionOrProperty>): CirType? {
         if (values.isEmpty()) return null
         val isTopLevel = values.all { it.containingClass == null }
-        return TypeCommonizer(classifiers, default.withCovariantNullabilityCommonizationEnabled(isTopLevel))
+        val isCovariant = values.none { it is CirProperty && it.isVar }
+        return TypeCommonizer(classifiers, default.withCovariantNullabilityCommonizationEnabled(isTopLevel && isCovariant))
             .asCommonizer().commonize(values.map { it.returnType })
     }
 }

native/commonizer/tests/org/jetbrains/kotlin/commonizer/hierarchical/ReturnTypeNullabilityCommonizationTest.kt

@@ -63,14 +63,33 @@ class ReturnTypeNullabilityCommonizationTest : AbstractInlineSourcesCommonizatio
     fun `test nullable and non-nullable - property`() {
         val result = commonize {
             outputTarget("(a, b)")
-            simpleSingleSourceTarget("a", "val x: Any? = Unit?")
+            simpleSingleSourceTarget("a", "val x: Any? = null")
             simpleSingleSourceTarget("b", "val x: Any = Unit")
         }
 
         result.assertCommonized("(a, b)", "expect val x: Any?")
     }
 
+    fun `test nullable and non-nullable - var - val property`() {
+        val result = commonize {
+            outputTarget("(a, b)")
+            simpleSingleSourceTarget("a", "var x: Any? = null")
+            simpleSingleSourceTarget("b", "val x: Any = Unit")
+        }
 
+        result.assertCommonized("(a, b)", "")
+    }
+
+    fun `test nullable and non-nullable - var var property`() {
+        val result = commonize {
+            outputTarget("(a, b)")
+            simpleSingleSourceTarget("a", "var x: Any? = null")
+            simpleSingleSourceTarget("b", "var x: Any = Unit")
+        }
+
+        result.assertCommonized("(a, b)", "")
+    }
+    
     fun `test different nullability typealias  - function`() {
         val result = commonize {
             outputTarget("(a, b)")