[Bug] undefined is required in a route's serialize method in the stable types

[boris-petrov]

🐞 Describe the Bug

Check the repro.

app/routes/foo.ts:8:19 - error TS2416: Property 'serialize' in type 'FooRoute<T>' is not assignable to the same property in base type 'Route<T>'.
  Type '(object: Child) => any' is not assignable to type '(model: T | undefined, params: string[]) => { [key: string]: unknown; } | undefined'.
    Types of parameters 'object' and 'model' are incompatible.
      Type 'T | undefined' is not assignable to type 'Child'.
        Type 'undefined' is not assignable to type 'Model'.

8   public override serialize(object: Child): any {
                    ~~~~~~~~~

🔬 Minimal Reproduction

Repo.

Run npm install and then ./node_modules/.bin/tsc.

😕 Actual Behavior

Type error on serialize.

🤔 Expected Behavior

No error. Removing the last commit (which upgrades Ember) and rerunning npm install and then tsc will remove the errors.

🌍 Environment

• Ember: 5.1.0
• Node.js/npm: 20.3.1/9.7.2
• OS: Linux
• Browser: N/A

cc @chriskrycho

[chriskrycho]

I have a fix for this at #20490, including a type test to make sure it does not regress. Note that the differences there are on purpose – your example code is actually invalid even with the fix, because the class cannot assume Child as the param, only T.

[boris-petrov]

Note that the differences there are on purpose – your example code is actually invalid even with the fix, because the class cannot assume Child as the param, only T.

Actually, it does compile with the fix (you can check by reverting the last commit). Which is rather strange, you're right, but I guess due to co/contravariance?

[chriskrycho]

At least in the context of the type tests I wrote, it does not type check. It's possible that the preview types themselves had a bug here which were allowing it!