Add empty enum as haxe.Unit

src-json/meta.json

@@ -738,6 +738,13 @@
 		"platforms": ["flash"],
 		"internal": true
 	},
+	{
+		"name": "Nullable",
+		"metadata": ":nullable",
+		"doc": "Explicitly admits `null` as valid value for an enum, which affects null-safety",
+		"targets": ["TEnum"],
+		"links": ["https://haxe.org/manual/types-nullability.html"]
+	},
 	{
 		"name": "NullSafety",
 		"metadata": ":nullSafety",

src/typing/nullSafety.ml

@@ -85,6 +85,8 @@ let rec is_nullable_type ?(dynamic_is_nullable=false) = function
 		is_nullable_type (apply_typedef t tl)
 	| (TDynamic _) as t ->
 		dynamic_is_nullable && t == t_dynamic
+	| TEnum(en,_) ->
+		Meta.has Meta.Nullable en.e_meta
 	| _ ->
 		false
 (*

std/haxe/Unit.hx

@@ -0,0 +1,29 @@
+/*
+ * Copyright (C)2005-2019 Haxe Foundation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+package haxe;
+
+/**
+	A unit type which can only be `null` at run-time.
+**/
+@:nullable
+enum Unit {}

tests/unit/src/unitstd/haxe/Unit.unit.hx

@@ -0,0 +1,9 @@
+var u:haxe.Unit = null;
+Reflect.isObject(u) == false;
+Reflect.isEnumValue(u) == false;
+Reflect.isFunction(u) == false;
+Reflect.compare(u, null) == 0;
+Reflect.compare(null, u) == 0;
+Reflect.compare(u, u) == 0;
+Type.getClass(u) == null;
+Type.getEnum(u) == null;