New default checks for mixed hierarchies

Old and new schemes

compiler/frontend/src/org/jetbrains/kotlin/resolve/LanguageVersionSettingsProvider.kt

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.resolve
+
+import com.intellij.openapi.components.ServiceManager
+import com.intellij.openapi.project.Project
+import org.jetbrains.kotlin.config.LanguageVersionSettings
+import org.jetbrains.kotlin.descriptors.ModuleDescriptor
+
+interface LanguageVersionSettingsProvider {
+
+    fun getModuleLanguageVersionSettings(module: ModuleDescriptor): LanguageVersionSettings?
+
+    companion object {
+        fun getInstance(project: Project): LanguageVersionSettingsProvider? =
+            ServiceManager.getService(project, LanguageVersionSettingsProvider::class.java)
+    }
+}

compiler/frontend/src/org/jetbrains/kotlin/util/declarationUtil.kt

@@ -98,13 +98,19 @@ fun getNonPrivateTraitMembersForDelegation(
     val result = linkedMapOf<CallableMemberDescriptor, CallableMemberDescriptor>()
     for (declaration in DescriptorUtils.getAllDescriptors(descriptor.defaultType.memberScope)) {
         if (declaration !is CallableMemberDescriptor) continue
-
-        val traitMember = findInterfaceImplementation(declaration, returnImplNotDelegate)
-        if (traitMember == null ||
-            Visibilities.isPrivate(traitMember.visibility) ||
-            traitMember.visibility == Visibilities.INVISIBLE_FAKE
-        ) continue
-        result[declaration] = traitMember
+        result[declaration] = getNonPrivateTraitMembersForDelegation(declaration, returnImplNotDelegate) ?: continue
     }
     return result
+}
+
+fun getNonPrivateTraitMembersForDelegation(
+    descriptor: CallableMemberDescriptor,
+    returnImplNotDelegate: Boolean = false,
+): CallableMemberDescriptor? {
+    val traitMember = findInterfaceImplementation(descriptor, returnImplNotDelegate)
+    if (traitMember == null ||
+        Visibilities.isPrivate(traitMember.visibility) ||
+        traitMember.visibility == Visibilities.INVISIBLE_FAKE
+    ) return null
+    return traitMember
 }