MaikM
Deno•8mo ago•
2 replies
Maik

Inconsistent type checking of overloaded generic functions on publish

Hi, thanks for taking the time to read this.

I'm running into inconsistent type checking behaviour of overloaded generic functions between 
deno check
 and 
deno publish
. I would expect both to exhibit the same kind of type checking behaviour but it seems I've found an edge-case.

The culprit function is defined as follows:
export function chain<R>(handler: Handler<R>): Handler<R>;
export function chain<R, P>(middleware: Middleware<R, P>): Chain<R, P>;
export function chain<R, P>(middleware: Handler<R> | Middleware<R, P>): Handler<R> | Chain<R, P> { ... }

Running 
deno check
 reports no type errors. However, using 
deno publish
 it reports the following error:
error: Failed ensuring public API type output is valid.

TS6205 [ERROR]: All type parameters are unused.
export function chain<R, P>(param0?: any): any {
                     ~~~~~~

The package exports defined in 
mod.ts
 have 
chain
 exported as a simple re-export: 
export { chain } from '...';
.

Some things I have learnt already:
- This is due to the 
noUnusedParameters
 compiler option, disabling said option removes the type error.
- The error is only reported when I export it as part of 
mod.ts
 (i.e. only when it is 'exported publicly').
- Using a 
@ts-ignore
 rule mitigates the type error.

Of course I could refactor to avoid the use of overloads but I figured this might be worth investigating.

Once again, thanks for taking the time to read into this. If anything remains unclear, I'd be more than happy to provide more information.
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