Convert to block body: specify non-nullable type when body expression is platform type and overriden method is non-nullable type

#KT-12222 Fixed

idea/src/org/jetbrains/kotlin/idea/intentions/ConvertToBlockBodyIntention.kt

@@ -18,8 +18,10 @@ import org.jetbrains.kotlin.psi.psiUtil.endOffset
 import org.jetbrains.kotlin.psi.psiUtil.startOffset
 import org.jetbrains.kotlin.types.KotlinType
 import org.jetbrains.kotlin.types.isError
+import org.jetbrains.kotlin.types.isNullabilityFlexible
 import org.jetbrains.kotlin.types.typeUtil.isNothing
 import org.jetbrains.kotlin.types.typeUtil.isUnit
+import org.jetbrains.kotlin.types.typeUtil.makeNotNullable
 
 class ConvertToBlockBodyIntention : SelfTargetingIntention<KtDeclarationWithBody>(
     KtDeclarationWithBody::class.java,
@@ -90,6 +92,13 @@ class ConvertToBlockBodyIntention : SelfTargetingIntention<KtDeclarationWithBody
             return declaration
         }
 
-        private fun KtNamedFunction.returnType(): KotlinType? = resolveToDescriptorIfAny()?.returnType
+        private fun KtNamedFunction.returnType(): KotlinType? {
+            val descriptor = resolveToDescriptorIfAny()
+            val returnType = descriptor?.returnType ?: return null
+            if (returnType.isNullabilityFlexible()
+                && descriptor.overriddenDescriptors.firstOrNull()?.returnType?.isMarkedNullable == false
+            ) return returnType.makeNotNullable()
+            return returnType
+        }
     }
 }

idea/testData/intentions/convertToBlockBody/overrideWithPlatformType.kt

@@ -0,0 +1,8 @@
+// WITH_RUNTIME
+interface Foo {
+    fun foo(): String
+}
+
+class Bar : Foo {
+    override fun <caret>foo() = java.lang.String.valueOf(42)
+}

idea/testData/intentions/convertToBlockBody/overrideWithPlatformType.kt.after

@@ -0,0 +1,10 @@
+// WITH_RUNTIME
+interface Foo {
+    fun foo(): String
+}
+
+class Bar : Foo {
+    override fun foo(): String {
+        return java.lang.String.valueOf(42)
+    }
+}

idea/tests/org/jetbrains/kotlin/idea/intentions/IntentionTestGenerated.java

@@ -6947,6 +6947,11 @@ public class IntentionTestGenerated extends AbstractIntentionTest {
             runTest("idea/testData/intentions/convertToBlockBody/nothingFun.kt");
         }
 
+        @TestMetadata("overrideWithPlatformType.kt")
+        public void testOverrideWithPlatformType() throws Exception {
+            runTest("idea/testData/intentions/convertToBlockBody/overrideWithPlatformType.kt");
+        }
+
         @TestMetadata("setter.kt")
         public void testSetter() throws Exception {
             runTest("idea/testData/intentions/convertToBlockBody/setter.kt");