Override/Implement: Prefer not-nullable return type when overriding Java method without nullability annotation

#KT-12381 Fixed

ChangeLog.md

@@ -392,6 +392,7 @@
 - [`KT-11328`](https://youtrack.jetbrains.com/issue/KT-11328) "New Kotlin class": generates packages when fully qualified name is specified
 - [`KT-11778`](https://youtrack.jetbrains.com/issue/KT-11778) Exception in Lombok plugin: Rewrite at slice FUNCTION
 - [`KT-11708`](https://youtrack.jetbrains.com/issue/KT-11708) "Go to declaration" doesn't work on a call to function with SAM conversion on a derived type
+- [`KT-12381`](https://youtrack.jetbrains.com/issue/KT-12381) Prefer not-nullable return type when overriding Java method without nullability annotation
 
 ### Reflection
 

idea/idea-core/src/org/jetbrains/kotlin/idea/core/overrideImplement/OverrideMemberChooserObject.kt

@@ -35,6 +35,7 @@ import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.psi.findDocComment.findDocComment
 import org.jetbrains.kotlin.renderer.*
 import org.jetbrains.kotlin.resolve.descriptorUtil.setSingleOverridden
+import org.jetbrains.kotlin.types.typeUtil.makeNotNullable
 
 interface OverrideMemberChooserObject : ClassMember {
     enum class BodyType {
@@ -155,9 +156,12 @@ private fun generateConstructorParameter(project: Project, descriptor: PropertyD
 }
 
 private fun generateFunction(project: Project, descriptor: FunctionDescriptor, bodyType: OverrideMemberChooserObject.BodyType): KtNamedFunction {
-    val newDescriptor = descriptor.copy(descriptor.containingDeclaration, Modality.OPEN, descriptor.visibility,
-                                        descriptor.kind, /* copyOverrides = */ true)
-    newDescriptor.setSingleOverridden(descriptor)
+    val newDescriptor = object : FunctionDescriptor by descriptor {
+        override fun getModality() = Modality.OPEN
+        override fun getReturnType() = descriptor.returnType?.makeNotNullable()
+        override fun getOverriddenDescriptors() = listOf(descriptor)
+        override fun <R : Any?, D : Any?> accept(visitor: DeclarationDescriptorVisitor<R, D>, data: D) = visitor.visitFunctionDescriptor(this, data)
+    }
 
     val returnType = descriptor.returnType
     val returnsNotUnit = returnType != null && !KotlinBuiltIns.isUnit(returnType)

idea/testData/codeInsight/overrideImplement/implementJavaRawSubclass/foo/Impl.kt.after

@@ -2,7 +2,7 @@ import foo.A
 import foo.B
 
 class C : A() {
-    override fun foo(x: MutableList<Any?>?, y: String?): B<*>? {
+    override fun foo(x: MutableList<Any?>?, y: String?): B<*> {
         <selection><caret>throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.</selection>
     }
 }

idea/testData/codeInsight/overrideImplement/javaInterfaceMethod/foo/Impl.kt.after

@@ -1,7 +1,7 @@
 import foo.Intf
 
 class Impl(): Intf {
-    override fun getFooBar(): String? {
+    override fun getFooBar(): String {
         <selection><caret>throw UnsupportedOperationException("not implemented") //To change body of created functions use File | Settings | File Templates.</selection>
     }
 }

idea/testData/codeInsight/overrideImplement/javaMethodWithPackageProtectedVisibility/foo/Impl.kt.after

@@ -3,7 +3,7 @@ package foo
 import foo.Intf
 
 class Impl(): Intf() {
-    override fun getFooBar(): String? {
+    override fun getFooBar(): String {
         <selection><caret>return super.getFooBar()</selection>
     }
 }

idea/testData/codeInsight/overrideImplement/javaMethodWithPackageVisibility/foo/Impl.kt.after

@@ -3,7 +3,7 @@ package foo
 import foo.Intf
 
 class Impl(): Intf() {
-    override fun getFooBar(): String? {
+    override fun getFooBar(): String {
         <selection><caret>return super.getFooBar()</selection>
     }
 }

idea/testData/codeInsight/overrideImplement/platformTypes/foo/A.java

@@ -0,0 +1,7 @@
+package foo;
+
+public class A {
+    public String foo(String s) {
+        return null;
+    }
+}

idea/testData/codeInsight/overrideImplement/platformTypes/foo/Impl.kt

@@ -0,0 +1,5 @@
+import foo.A
+
+class B : A() {
+    <caret>
+}

idea/testData/codeInsight/overrideImplement/platformTypes/foo/Impl.kt.after

@@ -0,0 +1,7 @@
+import foo.A
+
+class B : A() {
+    override fun foo(s: String?): String {
+        <selection><caret>return super.foo(s)</selection>
+    }
+}

idea/tests/org/jetbrains/kotlin/idea/codeInsight/OverrideImplementTest.kt

@@ -243,4 +243,8 @@ class OverrideImplementTest : AbstractOverrideImplementTest() {
     fun testConvertJavaDoc() {
         doOverrideDirectoryTest("foo")
     }
+
+    fun testPlatformTypes() {
+        doOverrideDirectoryTest("foo")
+    }
 }