[FIR] fix subtyping for nullable captured types.

The issue is the type checker doesn't consider P? a subtype of
CapturedType<in P>?, whereas P a subtype of CapturedType<in P>?. In
AbstractTypeCheckerContext::checkSubtypeForSpecialCases, it checks if
P? is a subtype of the lower type of the captured type, which is
P, and returns false.

This fix uses nullable version of the lower type when the captured
type is marked nullable. To check if P? is a subtype of Captured<in
P>?, we check the LHS, P?, against the nullable lower type of RHS,
P?.

^KT-42825 Fixed

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

@@ -407,9 +407,15 @@ object AbstractTypeChecker {
         val superTypeCaptured = superType.asCapturedType()
         val lowerType = superTypeCaptured?.lowerType()
         if (superTypeCaptured != null && lowerType != null) {
+            // If superType is nullable, e.g., to check if Foo? a subtype of Captured<in Foo>?, we check the LHS, Foo?,
+            // against the nullable version of the lower type of RHS. See KT-42825
+            val nullableLowerType =
+                if (superType.isMarkedNullable())
+                    lowerType.withNullability(true)
+                else lowerType
             when (getLowerCapturedTypePolicy(subType, superTypeCaptured)) {
-                CHECK_ONLY_LOWER -> return isSubtypeOf(this, subType, lowerType)
-                CHECK_SUBTYPE_AND_LOWER -> if (isSubtypeOf(this, subType, lowerType)) return true
+                CHECK_ONLY_LOWER -> return isSubtypeOf(this, subType, nullableLowerType)
+                CHECK_SUBTYPE_AND_LOWER -> if (isSubtypeOf(this, subType, nullableLowerType)) return true
                 SKIP_LOWER -> Unit
             }
         }

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

@@ -144,9 +144,6 @@ interface TypeSystemInferenceExtensionContext : TypeSystemContext, TypeSystemBui
 
     fun KotlinTypeMarker.isBuiltinFunctionalTypeOrSubtype(): Boolean
 
-    fun KotlinTypeMarker.withNullability(nullable: Boolean): KotlinTypeMarker
-
-
     fun KotlinTypeMarker.makeDefinitelyNotNullOrNotNull(): KotlinTypeMarker
     fun SimpleTypeMarker.makeSimpleTypeDefinitelyNotNullOrNotNull(): SimpleTypeMarker
 
@@ -241,7 +238,6 @@ interface TypeSystemContext : TypeSystemOptimizationContext {
     fun KotlinTypeMarker.isError(): Boolean
     fun TypeConstructorMarker.isError(): Boolean
     fun KotlinTypeMarker.isUninferredParameter(): Boolean
-
     fun FlexibleTypeMarker.asDynamicType(): DynamicTypeMarker?
 
     fun FlexibleTypeMarker.asRawType(): RawTypeMarker?
@@ -259,6 +255,7 @@ interface TypeSystemContext : TypeSystemOptimizationContext {
 
     fun SimpleTypeMarker.withNullability(nullable: Boolean): SimpleTypeMarker
     fun SimpleTypeMarker.typeConstructor(): TypeConstructorMarker
+    fun KotlinTypeMarker.withNullability(nullable: Boolean): KotlinTypeMarker
 
     fun CapturedTypeMarker.typeConstructor(): CapturedTypeConstructorMarker
     fun CapturedTypeMarker.captureStatus(): CaptureStatus