$ExpectType doesn't expand types?

[sharpchen]

I'm contributing to a DT package, sometimes I got message implying it doesn't compare types structurally:

1. eslint: TypeScript expected type to be:
     boolean | "yes or no"
   got:
     "yes or no" | NonNullable<boolean | undefined> [@definitelytyped/expect]

It's inconvenient for writing tests as the result type might be wrapped inside another generic in the future.
If the wrapper type is not a simple utility type, it can be confusing what exact type this test is expecting.

I know I can add & {} to the return type in the definition, but that would also make it confusing for maintainers.
And I did have a grep on the DT codebase, it turns out I would be the first one to do & {} for the return type 😅

[jakebailey]

The test is effectively like hovering. Do you see it unexpanded in a VS Code hover too?

[sharpchen]

Yeah lsp hover shows the same, sounds like typescript just can't emit final shape of the type?

[jakebailey]

I don't know the exact details but we choose not to expand things for various reasons, and this being an alias is probably one of them. I don't expect us to change dtslint to change this behavior because NonNullable is not special among other type aliases.

[santiagorojas14]

Since dtslint relies on the same internal printer as the language service, have you tried using a narrow helper like type Expand<T> = T extends infer U ? { [K in keyof U]: U[K] } : never in your test file to force the TS compiler to evaluate the mapped type before the assertion?