Method can be hidden if it is annotated with kotlin.internal.PlatformDependent,

or is a Java method from a Kotlin built-in class (transitively).

https://github.com/Kotlin/KEEP/blob/master/proposals/jdk-dependent-built-ins.md#backward-compatibility-of-overrides

compiler/testData/diagnostics/testsWithJava8/targetedBuiltIns/unsupportedFeature.kt

@@ -7,8 +7,7 @@ class A : java.util.ArrayList<String>() {
 }
 
 class A1 : java.util.ArrayList<String>() {
-    // TODO: should be allowed
-    <!VIRTUAL_MEMBER_HIDDEN!>fun stream(): java.util.stream.Stream<String><!> = super.<!DEPRECATION_ERROR!>stream<!>()
+    fun stream(): java.util.stream.Stream<String> = super.<!DEPRECATION_ERROR!>stream<!>()
 }
 
 class B : <!DEPRECATION_ERROR!>Throwable<!>("", null, false, false)