[JVM IR] Do not add nullability annotation for functions that return

platform types.

This behavior matches the non-IR backend, which was added in
f16dcdd8a9.

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/codegen/AnnotationCodegen.kt

@@ -32,10 +32,8 @@ import org.jetbrains.kotlin.descriptors.annotations.KotlinRetention
 import org.jetbrains.kotlin.descriptors.annotations.KotlinTarget
 import org.jetbrains.kotlin.ir.declarations.*
 import org.jetbrains.kotlin.ir.expressions.*
-import org.jetbrains.kotlin.ir.types.IrType
-import org.jetbrains.kotlin.ir.types.classifierOrNull
-import org.jetbrains.kotlin.ir.types.isMarkedNullable
-import org.jetbrains.kotlin.ir.types.isNullable
+import org.jetbrains.kotlin.ir.types.*
+import org.jetbrains.kotlin.ir.types.impl.originalKotlinType
 import org.jetbrains.kotlin.ir.util.*
 import org.jetbrains.kotlin.load.kotlin.FileBasedKotlinClass
 import org.jetbrains.kotlin.load.kotlin.KotlinJvmBinarySourceElement
@@ -45,6 +43,7 @@ import org.jetbrains.kotlin.resolve.checkers.ExpectedActualDeclarationChecker
 import org.jetbrains.kotlin.serialization.deserialization.descriptors.DeserializedClassDescriptor
 import org.jetbrains.kotlin.synthetic.isVisibleOutside
 import org.jetbrains.kotlin.types.TypeSystemCommonBackendContext
+import org.jetbrains.kotlin.types.isNullabilityFlexible
 import org.jetbrains.kotlin.types.model.KotlinTypeMarker
 import org.jetbrains.org.objectweb.asm.*
 import java.lang.annotation.RetentionPolicy
@@ -153,6 +152,11 @@ abstract class AnnotationCodegen(
             return
         }
 
+        // A flexible type whose lower bound in not-null and upper bound is nullable, should not be annotated
+        if (type.toKotlinType().isNullabilityFlexible()) {
+            return
+        }
+
         val annotationClass = if (type.isNullable()) Nullable::class.java else NotNull::class.java
 
         generateAnnotationIfNotPresent(annotationDescriptorsAlreadyPresent, annotationClass)

compiler/testData/codegen/bytecodeListing/platformTypes.kt

@@ -0,0 +1,8 @@
+// WITH_RUNTIME
+import java.util.Collections
+
+class PlatformTypes {
+    fun simplyPlatform() = Collections.singletonList("")[0]
+    fun bothNullable() = Collections.emptyList<String>() ?: null
+    fun bothNotNull() = Collections.emptyList<String>()!!
+}

compiler/testData/codegen/bytecodeListing/platformTypes.txt

@@ -0,0 +1,7 @@
+@kotlin.Metadata
+public final class PlatformTypes {
+    public method <init>(): void
+    public final @org.jetbrains.annotations.NotNull method bothNotNull(): java.util.List
+    public final @org.jetbrains.annotations.Nullable method bothNullable(): java.util.List
+    public final method simplyPlatform(): java.lang.String
+}

compiler/tests/org/jetbrains/kotlin/codegen/BytecodeListingTestGenerated.java

@@ -94,6 +94,11 @@ public class BytecodeListingTestGenerated extends AbstractBytecodeListingTest {
         runTest("compiler/testData/codegen/bytecodeListing/noToArrayInJava.kt");
     }
 
+    @TestMetadata("platformTypes.kt")
+    public void testPlatformTypes() throws Exception {
+        runTest("compiler/testData/codegen/bytecodeListing/platformTypes.kt");
+    }
+
     @TestMetadata("privateDefaultSetter.kt")
     public void testPrivateDefaultSetter() throws Exception {
         runTest("compiler/testData/codegen/bytecodeListing/privateDefaultSetter.kt");