Fails to type check annotated code

Here is an example of the code:

```ts
type Just = {
x: T
}
enum Maybe {
None,
Just
}

const delta = (a:int, b:int):Maybe => {
if a == b {
None
} else {
Just{x: b - a}
}
}
```

Only thing that seems to works is

Start by reproducing the type-checking failure with the annotated TypeScript example in the issue and compare it with the working form shown in the final image. Trace the type-checking entry point and identify the change needed for the example to type-check successfully.

Written by the indexing model from the issue text.