Unification error regarding abstracts

[kevinresol]

I tried but can't reduce it to not depend on a library. I am not sure what's happening behind the hood.
(Not regression)

-lib tink_core

package;

using tink.CoreApi;

class Main {
    public static function main() {

        var a:Surprise<String, ErrorAbstract> = null;
        var r:Response = a; // compiler complains: tink.Surprise<String, ErrorAbstract> should be Response
    }
}


typedef Rep = Surprise<String, Error>;
abstract Response(Rep) from Rep to Rep {}
abstract ErrorAbstract(Error) from Error to Error {}

[kevinresol]

Not sure if it is related, here is another problem.
The following code does not compile:

package;

class Main {
    public static function main() {
        function test() //:Int (works if explicitly hint return type as Int)
            return if (true) getInt() else getAbstract();
    }

    static function getInt():Int return 1;
    static function getAbstract():MyAbstract return "1";
}
abstract MyAbstract(String) from String to String {
    @:to
    public inline function toInt():Int return Std.parseInt(this);
}

[RealyUniqueName]

Original issue described in start post is about abstracts transitive casts. Let's track it here: #5997
For the second post I created a separate issue: #8479