J2K: Remove unnecessary casts of non-nullable expression to nullable type after conversion

#KT-11543 Fixed

idea/src/org/jetbrains/kotlin/idea/j2k/J2kPostProcessings.kt

@@ -21,6 +21,7 @@ import com.intellij.psi.search.searches.ReferencesSearch
 import org.jetbrains.kotlin.diagnostics.Diagnostic
 import org.jetbrains.kotlin.diagnostics.DiagnosticFactory
 import org.jetbrains.kotlin.diagnostics.Errors
+import org.jetbrains.kotlin.idea.caches.resolve.analyzeAndGetResult
 import org.jetbrains.kotlin.idea.core.setVisibility
 import org.jetbrains.kotlin.idea.inspections.RedundantSamConstructorInspection
 import org.jetbrains.kotlin.idea.intentions.*
@@ -34,6 +35,7 @@ import org.jetbrains.kotlin.idea.references.mainReference
 import org.jetbrains.kotlin.lexer.KtTokens
 import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.psi.psiUtil.visibilityModifierType
+import org.jetbrains.kotlin.resolve.BindingContext
 import org.jetbrains.kotlin.resolve.diagnostics.Diagnostics
 import java.util.*
 
@@ -54,6 +56,7 @@ object J2KPostProcessingRegistrar {
         _processings.add(ConvertToStringTemplateProcessing())
         _processings.add(UsePropertyAccessSyntaxProcessing())
         _processings.add(RemoveRedundantSamAdaptersProcessing())
+        _processings.add(RemoveRedundantCastToNullableProcessing())
 
         registerIntentionBasedProcessing(ConvertToExpressionBodyIntention()) { it is KtPropertyAccessor }
         registerIntentionBasedProcessing(IfThenToSafeAccessIntention())
@@ -219,4 +222,23 @@ object J2KPostProcessingRegistrar {
             }
         }
     }
+
+    private class RemoveRedundantCastToNullableProcessing : J2kPostProcessing {
+        override fun createAction(element: KtElement, diagnostics: Diagnostics): (() -> Unit)? {
+            if (element !is KtBinaryExpressionWithTypeRHS) return null
+
+            val context = element.analyzeAndGetResult().bindingContext
+            val leftType = context.getType(element.left) ?: return null
+            val rightType = context.get(BindingContext.TYPE, element.right) ?: return null
+
+            if (!leftType.isMarkedNullable && rightType.isMarkedNullable) {
+                return {
+                    val type = element.right?.typeElement as? KtNullableType
+                    type?.replace(type.innerType!!)
+                }
+            }
+
+            return null
+        }
+    }
 }

j2k/testData/fileOrElement/nullability/notNullCast.java

@@ -0,0 +1,32 @@
+public class Passenger {
+    public static class PassParent {
+    }
+
+    public static class PassChild extends PassParent {
+    }
+
+    public PassParent provideNullable(int p) {
+        return p > 0 ? new PassChild() : null;
+    }
+
+    public void test1() {
+        PassParent pass = provideNullable(1);
+        assert pass != null;
+        accept1((PassChild) pass);
+    }
+
+    public void test2() {
+        PassParent pass = provideNullable(1);
+        if (1 == 2) {
+            assert pass != null;
+            accept2((PassChild) pass);
+        }
+        accept2((PassChild) pass);
+    }
+
+    public void accept1(PassChild p) {
+    }
+
+    public void accept2(PassChild p) {
+    }
+}

j2k/testData/fileOrElement/nullability/notNullCast.kt

@@ -0,0 +1,31 @@
+// ERROR: Type mismatch: inferred type is Passenger.PassChild? but Passenger.PassChild was expected
+// ERROR: Type mismatch: inferred type is Passenger.PassChild? but Passenger.PassChild was expected
+class Passenger {
+    open class PassParent
+
+    class PassChild : PassParent()
+
+    fun provideNullable(p: Int): PassParent? {
+        return if (p > 0) PassChild() else null
+    }
+
+    fun test1() {
+        val pass = provideNullable(1)!!
+        accept1(pass as PassChild)
+    }
+
+    fun test2() {
+        val pass = provideNullable(1)
+        if (1 == 2) {
+            assert(pass != null)
+            accept2(pass as PassChild?)
+        }
+        accept2(pass as PassChild?)
+    }
+
+    fun accept1(p: PassChild) {
+    }
+
+    fun accept2(p: PassChild) {
+    }
+}

j2k/tests/org/jetbrains/kotlin/j2k/JavaToKotlinConverterForWebDemoTestGenerated.java

@@ -3505,6 +3505,12 @@ public class JavaToKotlinConverterForWebDemoTestGenerated extends AbstractJavaTo
             doTest(fileName);
         }
 
+        @TestMetadata("notNullCast.java")
+        public void testNotNullCast() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("j2k/testData/fileOrElement/nullability/notNullCast.java");
+            doTest(fileName);
+        }
+
         @TestMetadata("nullableField.java")
         public void testNullableField() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("j2k/testData/fileOrElement/nullability/nullableField.java");

j2k/tests/org/jetbrains/kotlin/j2k/JavaToKotlinConverterSingleFileTestGenerated.java

@@ -3505,6 +3505,12 @@ public class JavaToKotlinConverterSingleFileTestGenerated extends AbstractJavaTo
             doTest(fileName);
         }
 
+        @TestMetadata("notNullCast.java")
+        public void testNotNullCast() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("j2k/testData/fileOrElement/nullability/notNullCast.java");
+            doTest(fileName);
+        }
+
         @TestMetadata("nullableField.java")
         public void testNullableField() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("j2k/testData/fileOrElement/nullability/nullableField.java");