[Core API] Introduce TypeRefinement annotations

core/descriptors/src/org/jetbrains/kotlin/types/refinement/annotations.kt

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2010-2019 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.types.refinement
+
+/**
+ * This annotation marks part of internal compiler API related to type refinement.
+ *
+ * Marking such API explicitly has two objectives:
+ * - prevent unconscious abuse of invasive API (like DelegatingSimpleType.replaceDelegate),
+ *   which shouldn't be needed by anything outside of type refinement
+ * - improve readability of classes by separating API
+ *
+ * If you're using related API outside of MPP context, it's a nice idea to consider
+ * either finding some other API or removing @TypeRefinement (and thus "publishing"
+ * API for broader use)
+ */
+@Experimental(level = Experimental.Level.ERROR)
+@Retention(AnnotationRetention.BINARY)
+@Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION, AnnotationTarget.PROPERTY)
+annotation class TypeRefinement