Type parameter variance of enum

proposals/0000-variance-of-enum.md

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+# Type parameter variance of enum
+
+* Proposal: HXP-NNNN
+* Author: [shohei909](https://github.com/shohei909)
+
+## Introduction
+
+Supports type parameter covariance and contravariance of enum.
+
+## Motivation
+
+A common case is to assign an `Option` to another `Option`.
+
+```haxe
+import haxe.ds.Option;
+
+class Main {
+	static public function main() {
+		var optionFloat:Option<Float>;
+		var optionInt = Option.Some(1);
+		optionFloat = optionInt;
+	}
+}
+```
+
+Currently, following compile errors occur.
+
+```
+haxe.ds.Option<Int> should be haxe.ds.Option<Float>
+Type parameters are invariant
+Int should be Float
+```
+
+There are two methods to avoid this. The first is `cast`.
+
+```haxe
+var option:Option<Float> = cast Option.Some(1);
+```
+
+However, `cast` is not safe.
+
+The second is re-instantiating the `Option`.
+
+```haxe
+class OptionTools {
+    public static function upcast<T, U:T>(option:Option<U>):Option<T> {
+        return switch (option) {
+            case Option.Some(v): Option.Some((v:T));
+            case Option.None:    Option.None;
+        }
+    }
+}
+```
+
+```haxe
+var option:Option<Float> = OptionTools.upcast(Option.Some(1));
+```
+
+That is safe, but introduces runtime overhead.
+
+If covariance is supported, `Option<Int>` can be `Option<Float>`.
+
+### Actual example
+
+This is important when using enum with type parameter and interface together.
+
+Here is an example of defining `DummyUser` class for testing.
+
+```haxe
+interface IUser {
+    var profile(default, never):Option<IProfile>; 
+}
+
+interface IProfile {}
+
+class DummyUser implements IUser {
+    // If Option<DummyProfile> is not Option<IProfile>, the compile error occurs.
+    public var profile(default, null):Option<DummyProfile>;
+}
+
+class DummyProfile implements IProfile {}
+```
+
+## Detailed design
+
+In terms of variance, the arguments of each constructor of enum are same as the get-only properties. 
+
+### Contravariance
+
+The following Sample enum can be contravariant.
+
+```haxe
+enum Sample<T> {
+    A(T->Void);
+}
+```
+
+```haxe
+var sample:Sample<Int> = Sample(function (f:Float) {});
+```
+
+### Compiler output
+
+In dynamic targets, the compiler output is the same as when `cast`. 
+
+In the current static targets (Flash, C++, C#, Java), enum type parameter seems to be disappeared from output, so it should be same.
+
+### GADT
+
+Variance of GADT(generalized algebraic data type) can break exisiting code.
+
+```haxe
+enum Gadt<T> {
+    I(i:Int):Gadt<Int>;
+    F(f:Float):Gadt<Float>;
+}
+```
+
+`Gadt<Float>` doesn't have `I` constructor. The following code causes no compile error.
+
+```haxe
+switch (Gadt.F(0.0)) {
+    case Gadt.F(_):
+}
+```
+
+If `Gadt<T>` is covariant, `Gadt<Float>` should have `I` constructor.
+
+Therefore, I think that GADT should be invariant at least by default.
+
+## Impact on existing code
+
+No impact on existing code.
+
+## Alternatives
+
+Alternatives are described in [Motivation](#motivation) section.
+
+## Drawbacks
+
+This may make some feature additions difficult, such as [Enum member method/variables](https://github.com/HaxeFoundation/haxe-evolution/issues/10).