Callback in mapped type implicitly has 'any' type

TypeScript Version: 2.9.0-dev.20180414

Code

declare function watch<T>(obj: T, propertyChangedCallbacks: { [K in keyof T]: (obj: T) => void }): void;
watch({ x: 0 }, {
    x: obj => {},
});

Expected behavior:

obj is { x: number }.

Actual behavior:

src/a.ts(3,8): error TS7006: Parameter 'obj' implicitly has an 'any' type.

This is happening on 2.8.1 too:

type Events = {
  ready: () => void
  error: (e: Error) => void
  reconnecting: (params: { attempt: number, delay: number}) => void
}

type RedisClient = {
  on<E extends keyof Events>(event: E, f: Events[E]): void
}

let c: RedisClient

c.on('ready', () => 4)               // BAD - arity is propagated, return type is any
c.on('error', e => 4)                // BAD - e is any
c.on('reconnecting', p => p.attempt) // BAD - p is any

Playground

I just checked this in the TS playground, looks like this has been fixed (since v3.5.1)

Confirmed

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Callback in mapped type implicitly has 'any' type #23429

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Callback in mapped type implicitly has 'any' type #23429

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bcherny commented on Apr 22, 2018

russelldavis commented on Jun 5, 2023

RyanCavanaugh commented on Jul 23, 2025

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