Introduce Type Alias: Don't change non-nullable type to nullable alias

#KT-15840 Fixed

idea/src/org/jetbrains/kotlin/idea/refactoring/introduce/introduceTypeAlias/introduceTypeAliasImpl.kt

@@ -45,6 +45,7 @@ import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.psi.psiUtil.*
 import org.jetbrains.kotlin.resolve.BindingContext
 import org.jetbrains.kotlin.resolve.scopes.utils.findClassifier
+import org.jetbrains.kotlin.types.TypeUtils
 import java.util.*
 
 sealed class IntroduceTypeAliasAnalysisResult {
@@ -214,6 +215,9 @@ fun findDuplicates(typeAlias: KtTypeAlias): Map<KotlinPsiRange, () -> Unit> {
                 else continue
             }
             if (arguments.size != typeAliasDescriptor.declaredTypeParameters.size) continue
+            if (TypeUtils.isNullableType(typeAliasDescriptor.underlyingType)
+                && occurrence is KtUserType
+                && occurrence.parent !is KtNullableType) continue
             rangesWithReplacers += occurrence.toRange() to { replaceOccurrence(occurrence, arguments) }
         }
     }

idea/testData/refactoring/introduceTypeAlias/unmatchedNullability.kt

@@ -0,0 +1,5 @@
+// NAME: NullableA
+
+class A
+
+fun f(p: A): <selection>A?</selection> = null

idea/testData/refactoring/introduceTypeAlias/unmatchedNullability.kt.after

@@ -0,0 +1,7 @@
+// NAME: NullableA
+
+class A
+
+typealias NullableA = A?
+
+fun f(p: A): NullableA = null

idea/tests/org/jetbrains/kotlin/idea/refactoring/introduce/ExtractionTestGenerated.java

@@ -4268,6 +4268,12 @@ public class ExtractionTestGenerated extends AbstractExtractionTest {
             doIntroduceTypeAliasTest(fileName);
         }
 
+        @TestMetadata("unmatchedNullability.kt")
+        public void testUnmatchedNullability() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("idea/testData/refactoring/introduceTypeAlias/unmatchedNullability.kt");
+            doIntroduceTypeAliasTest(fileName);
+        }
+
         @TestMetadata("userTypeDuplicatesNoTypeParameters.kt")
         public void testUserTypeDuplicatesNoTypeParameters() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("idea/testData/refactoring/introduceTypeAlias/userTypeDuplicatesNoTypeParameters.kt");