Test for delegated members being overridden

compiler/testData/diagnostics/tests/override/Delegation.kt

@@ -0,0 +1,49 @@
+package test
+
+trait X {
+    fun foo(): String? {
+        return null
+    }
+}
+
+trait Y {
+    fun foo(): String {
+        return "foo"
+    }
+}
+
+trait Incompatible {
+    fun foo(): Int {
+        return 3
+    }
+}
+
+class Test1(val x: X) : X by x, Y {
+    override fun foo(): <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>String?<!> {
+        return null
+    }
+}
+
+class Test2(val x: X) : X by x, Y {
+    override fun foo(): String {
+        return "foo"
+    }
+}
+
+class Test3(val y: Y) : X, Y by y {
+    override fun foo(): <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>String?<!> {
+        return null
+    }
+}
+
+class Test4(val y: Y) : X, Y by y {
+    override fun foo(): String {
+        return "foo"
+    }
+}
+
+class Test5(val y: Y, val x: X) : X by x, Y by y, Incompatible {
+    override fun foo(): <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>Int<!> {
+        return 3
+    }
+}