add test (closes #6215)

tests/unit/src/unit/issues/Issue6215.hx

@@ -0,0 +1,17 @@
+package unit.issues;
+
+class Issue6215 extends unit.Test {
+	function test() {
+		var a:Action<Issue6215> = null;
+		var signal:Abstr<Child> = null;
+		signal += a;
+	}
+}
+
+private class Child extends Issue6215 {}
+
+private typedef Action<T> = T->Void;
+
+private abstract Abstr<T>(Dynamic) {
+	@:op(x += y) function opAdd(fn:Action<T>) return this;
+}