Generic type for an interface method is infered as any

[robak86]

TypeScript Version: 3.9.2

Search Terms:
interface generic any, interface method inference

Code

export interface IMiddleware<TInput, TOutput> {
  run(input: TInput): TOutput;
}

type Input = { x: number };
type Output = { y: number };

class MiddlewareImpl implements IMiddleware<Input, Output> {
  run(input) {      // input should be infered as Input instead of any (since Input is a supertype for input param)
    return input    // error should be displayed - {x: number} is not assignable to {y: number} 
  }
}

const classMiddleware = new MiddlewareImpl(); // classMiddleware is actually IMiddleware<any, any> instead of IMiddleware<Input, Output> even if it's clearly annotated that MiddlewareImpl implements (at least) IMiddleware<Input, Output>
classMiddleware.run({});                      // error should be thrown: Property 'x' is missing in type '{}' but required in type 'Input'     

Expected behavior:
The generic type of the implemented interface shouldn't be replaced with any, if the implemented method doesn't explicitly widens it to any

Actual behavior:
If a class implementing a given interface doesn't explicitly define types for a method parameter that uses generic types passed to the interface, then the parameter is cast to any, making the final type wider than expected.

I wasn't sure if this shouldn't be reported rather as a "feature request", but I finally chose bug, since I found this behavior highly unintuitive for somebody who does not use noImplicitAny

Playground Link: https://www.typescriptlang.org/play/?noImplicitAny=false#code/KYDwDg9gTgLgBASwHY2FAZgQwMbDgSQFkEATEgG2AHdMpgAeAFXyTAFcYAaORgeQ-YwAfHADeAKDhwobJAApkggFw8WggJQq+AjgG5xAX3HiYATzB41HOAF4xcECqRsAtgCM0cA-rMW4-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

Related Issues: -

[DanielRosenwasser]

Frustrating, but a duplicate of #23911

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.