[NI] Preserve nullability of resulting type from CST if it's possible

Consider common supertype of `S` and `Nothing`, where `S` has nullable
 upper bound or it's flexible. Before the fix, result was `S?`, which
 is correct but too conservative. Now, we'll preserve nullability of
 resulting type if it's already nullable.

 This happened because we were failing to find path of not-nullable
 types from `Nothing` to `S`, which should obviously exists by
 semantics of Nothing

compiler/testData/diagnostics/tests/inference/commonSystem/cstFromNullableChildAndNonParameterizedType.kt

@@ -0,0 +1,20 @@
+// !LANGUAGE: +NewInference
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+interface Base<out T>
+
+class ParameterizedChild<out R> : Base<R>
+class Child : Base<Nothing>
+
+fun <K> elvis(x: K?, y: K): K = TODO()
+fun <K> select(x: K, y: K): K = TODO()
+
+fun <V> myRun(f: () -> V): V = f()
+
+fun <S> test1(a: ParameterizedChild<S>?, b: Child): Base<S> = myRun {
+    elvis(a, b)
+}
+
+fun <S> test2(a: S?, b: S): S = <!TYPE_MISMATCH!>myRun {
+    <!TYPE_MISMATCH!>select(a, b)<!>
+}<!>

compiler/testData/diagnostics/tests/inference/commonSystem/cstFromNullableChildAndNonParameterizedType.txt

@@ -0,0 +1,27 @@
+package
+
+public fun </*0*/ K> elvis(/*0*/ x: K?, /*1*/ y: K): K
+public fun </*0*/ V> myRun(/*0*/ f: () -> V): V
+public fun </*0*/ K> select(/*0*/ x: K, /*1*/ y: K): K
+public fun </*0*/ S> test1(/*0*/ a: ParameterizedChild<S>?, /*1*/ b: Child): Base<S>
+public fun </*0*/ S> test2(/*0*/ a: S?, /*1*/ b: S): S
+
+public interface Base</*0*/ out T> {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class Child : Base<kotlin.Nothing> {
+    public constructor Child()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class ParameterizedChild</*0*/ out R> : Base<R> {
+    public constructor ParameterizedChild</*0*/ out R>()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -9823,6 +9823,11 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                     runTest("compiler/testData/diagnostics/tests/inference/commonSystem/boundOnNullableVariable.kt");
                 }
 
+                @TestMetadata("cstFromNullableChildAndNonParameterizedType.kt")
+                public void testCstFromNullableChildAndNonParameterizedType() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/inference/commonSystem/cstFromNullableChildAndNonParameterizedType.kt");
+                }
+
                 @TestMetadata("dontCaptureTypeVariable.kt")
                 public void testDontCaptureTypeVariable() throws Exception {
                     runTest("compiler/testData/diagnostics/tests/inference/commonSystem/dontCaptureTypeVariable.kt");

compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java

@@ -9818,6 +9818,11 @@ public class DiagnosticsUsingJavacTestGenerated extends AbstractDiagnosticsUsing
                     runTest("compiler/testData/diagnostics/tests/inference/commonSystem/boundOnNullableVariable.kt");
                 }
 
+                @TestMetadata("cstFromNullableChildAndNonParameterizedType.kt")
+                public void testCstFromNullableChildAndNonParameterizedType() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/inference/commonSystem/cstFromNullableChildAndNonParameterizedType.kt");
+                }
+
                 @TestMetadata("dontCaptureTypeVariable.kt")
                 public void testDontCaptureTypeVariable() throws Exception {
                     runTest("compiler/testData/diagnostics/tests/inference/commonSystem/dontCaptureTypeVariable.kt");

core/descriptors/src/org/jetbrains/kotlin/types/checker/NewKotlinTypeChecker.kt

@@ -30,6 +30,7 @@ import org.jetbrains.kotlin.types.checker.TypeCheckerContext.SeveralSupertypesWi
 import org.jetbrains.kotlin.types.checker.TypeCheckerContext.SupertypesPolicy
 import org.jetbrains.kotlin.types.typeUtil.asTypeProjection
 import org.jetbrains.kotlin.types.typeUtil.isAnyOrNullableAny
+import org.jetbrains.kotlin.types.typeUtil.isNothing
 import org.jetbrains.kotlin.types.typeUtil.makeNullable
 import org.jetbrains.kotlin.utils.SmartList
 import org.jetbrains.kotlin.utils.addToStdlib.safeAs
@@ -452,7 +453,7 @@ object NullabilityChecker {
             }
 
     private fun TypeCheckerContext.hasPathByNotMarkedNullableNodes(start: SimpleType, end: TypeConstructor) =
-            anySupertype(start, { !it.isMarkedNullable && it.constructor == end }) {
+            anySupertype(start, { it.isNothing() || (!it.isMarkedNullable && it.constructor == end) }) {
                 if (it.isMarkedNullable) SupertypesPolicy.None else SupertypesPolicy.LowerIfFlexible
             }