Preserve nullability while substituting captured types

compiler/testData/capturedTypeApproximation/nullableTypeVariable.kt

@@ -0,0 +1,11 @@
+class Inv<T>
+
+fun foo<T>(a: Inv<T?>) {}
+
+// T captures 'in Int'
+// lower: Nothing
+// upper: Inv<in Int?>
+
+// T captures 'out Int'
+// lower: Nothing
+// upper: Inv<out Int?>

compiler/testData/diagnostics/tests/inference/capturedTypes/captureForNullableTypes.kt

@@ -0,0 +1,24 @@
+// !DIAGNOSTICS: -UNUSED_VARIABLE -UNUSED_PARAMETER
+// !CHECK_TYPE
+
+fun <T: Any> bar(a: Array<T>): Array<T?> =  null!!
+
+fun test1(a: Array<Int>) {
+    val r: Array<Int?> = bar(a)
+    val t = bar(a)
+    t checkType { it : _<Array<Int?>> }
+}
+
+fun <T: Any> foo(l: Array<T>): Array<Array<T?>> = null!!
+
+fun test2(a: Array<out Int>) {
+    val r: Array<out Array<out Int?>> = foo(a)
+    val t = foo(a)
+    t checkType { it : _<Array<out Array<out Int?>>> }
+}
+
+fun test3(a: Array<in Int>) {
+    val r: Array<out Array<in Int?>> = foo(a)
+    val t = foo(a)
+    t checkType { it : _<Array<out Array<in Int?>>> }
+}

compiler/testData/diagnostics/tests/inference/capturedTypes/captureForNullableTypes.txt

@@ -0,0 +1,7 @@
+package
+
+internal fun </*0*/ T : kotlin.Any> bar(/*0*/ a: kotlin.Array<T>): kotlin.Array<T?>
+internal fun </*0*/ T : kotlin.Any> foo(/*0*/ l: kotlin.Array<T>): kotlin.Array<kotlin.Array<T?>>
+internal fun test1(/*0*/ a: kotlin.Array<kotlin.Int>): kotlin.Unit
+internal fun test2(/*0*/ a: kotlin.Array<out kotlin.Int>): kotlin.Unit
+internal fun test3(/*0*/ a: kotlin.Array<in kotlin.Int>): kotlin.Unit