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/backwardCompatibility/delegation.kt

@@ -0,0 +1,32 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER -PLATFORM_CLASS_MAPPED_TO_KOTLIN
+
+
+// Default methods from Java interfaces are NOT overridden by delegation.
+// So, in the example below 'TestNoDelegationToDefaultMethods#replace' implicitly overrides a method from 'java.util.Map' (which is ok),
+// but not a method from 'WithDelegation' (would be an error).
+open class WithDelegation(val m: Map<String, String>) : Map<String, String> by m
+
+class TestNoDelegationToDefaultMethods(m: Map<String, String>): WithDelegation(m) {
+    <!VIRTUAL_MEMBER_HIDDEN!>fun containsKey(key: String): Boolean<!> = TODO()
+
+    fun getOrDefault(key: String, defaultValue: String): String = TODO()
+
+    fun replace(key: String, value: String): String? = TODO()
+}
+
+
+
+interface IBaseWithKotlinDeclaration : Map<String, String> {
+    fun replace(key: String, value: String): String?
+}
+
+abstract class WithDelegation2(val m: Map<String, String>) : Map<String, String> by m, IBaseWithKotlinDeclaration
+
+abstract class TestNoDelegationToDefaultMethods2(m: Map<String, String>): WithDelegation2(m) {
+    <!VIRTUAL_MEMBER_HIDDEN!>fun containsKey(key: String): Boolean<!> = TODO()
+
+    fun getOrDefault(key: String, defaultValue: String): String = TODO()
+
+    // VIRTUAL_MEMBER_HIDDEN: hides member declaration inherited from a Kotlin interface
+    <!VIRTUAL_MEMBER_HIDDEN!>fun replace(key: String, value: String): String?<!> = TODO()
+}