[Commonizer] Fix: Lost nullability in commonized type aliases

native/commonizer/src/org/jetbrains/kotlin/descriptors/commonizer/cir/factory/CirTypeFactory.kt

@@ -115,6 +115,26 @@ object CirTypeFactory {
         )
     }
 
+    fun makeNullable(classOrTypeAliasType: CirClassOrTypeAliasType): CirClassOrTypeAliasType =
+        if (classOrTypeAliasType.isMarkedNullable)
+            classOrTypeAliasType
+        else
+            when (classOrTypeAliasType) {
+                is CirClassType -> createClassType(
+                    classId = classOrTypeAliasType.classifierId,
+                    outerType = classOrTypeAliasType.outerType,
+                    visibility = classOrTypeAliasType.visibility,
+                    arguments = classOrTypeAliasType.arguments,
+                    isMarkedNullable = true
+                )
+                is CirTypeAliasType -> createTypeAliasType(
+                    typeAliasId = classOrTypeAliasType.classifierId,
+                    underlyingType = makeNullable(classOrTypeAliasType.underlyingType),
+                    arguments = classOrTypeAliasType.arguments,
+                    isMarkedNullable = true
+                )
+            }
+
     private fun createClassTypeWithAllOuterTypes(
         classDescriptor: ClassDescriptor,
         arguments: List<CirTypeProjection>,

native/commonizer/src/org/jetbrains/kotlin/descriptors/commonizer/core/TypeCommonizer.kt

@@ -140,13 +140,19 @@ private class TypeAliasTypeCommonizer(private val cache: CirClassifiersCache) :
 
             // type alias don't needs to be commonized because it is from the standard library
             fun forKnownUnderlyingType(underlyingType: CirClassOrTypeAliasType) = object : CommonizedTypeAliasTypeBuilder {
-                override fun build(typeAliasId: ClassId, arguments: List<CirTypeProjection>, isMarkedNullable: Boolean) =
+                override fun build(typeAliasId: ClassId, arguments: List<CirTypeProjection>, isMarkedNullable: Boolean): CirTypeAliasType {
-                    CirTypeFactory.createTypeAliasType(
+                    val underlyingTypeWithProperNullability = if (isMarkedNullable && !underlyingType.isMarkedNullable)
+                        CirTypeFactory.makeNullable(underlyingType)
+                    else
+                        underlyingType
+
+                    return CirTypeFactory.createTypeAliasType(
                         typeAliasId = typeAliasId,
-                        underlyingType = underlyingType, // TODO replace arguments???
+                        underlyingType = underlyingTypeWithProperNullability, // TODO replace arguments???
                         arguments = arguments,
                         isMarkedNullable = isMarkedNullable
                     )
+                }
             }
         }
     }