regression in 3.1+: mapped type cannot be used to index type #27413

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1. weswigham commented on Sep 28, 2018

   

   Wesley Wigham (weswigham)

   on Sep 28, 2018

   Last edited by weswigham

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   interface Table<T> {
   	select<U extends {[K: string]: keyof T}>
   	      (fields: U): {[K in keyof U]: T[U[K]]}
   }
   
   declare var t: Table<{x: string}>;
   declare const s: unique symbol;
   // your definition of `U` allows symbolic keys of `U` to have non-`keyof T` types,
   // which in turn means a call like this would typecheck. `T` indexed by one of these
   // types wouldn't exist, which is why we issue this error
   const result = t.select({[s]: "not a key", member: "x"});
   // `result` above will end up of type `{[s]: unknown, member: string}`, since your type doesn't
   // actually handle the `symbol` key case

   I assume, anyway. I actually tried what I think would be the correct version that fixes that, though:

   interface Table<T> {
   	select<U extends {[K: string]: keyof T}>
   	      (fields: U): {[K in Extract<keyof U, string>]: T[U[K]]}
   }

   and had the same issue; so something else must be the issue.

   In any case, you can work around the problem by writing your type like so:

   interface Table<T> {
   	select<U extends {[K: string]: keyof T}>
   	      (fields: U): {[K in Extract<keyof U, string>]: T[U[K] & keyof T]}
   }

   (I left in the Extract so symbols on the input type get ignored, and the & keyof T in the index is what makes it typecheck despite the issue. It actually should have absolutely no effect on the resulting types when the generics are instantiated; but it shouldn't be necessary since the constraint of U[K] when K extends Extract<keyof U, string> aught to be keyof T as you implied.)
2. yseymour commented on Sep 28, 2018

   

   Yudhi Seymour (yseymour)

   on Sep 28, 2018

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   Thanks, good call on the workaround. U[K] used to be implicitly constrained to keyof T. I'm not sure exactly when the behaviour changed.
3. Ryan Cavanaugh (RyanCavanaugh)

   added

   BugA bug in TypeScriptA bug in TypeScript

   on Oct 1, 2018
4. Ryan Cavanaugh (RyanCavanaugh)

   added this to the Future milestone on Oct 1, 2018
5. Carlos Rodrigues (pikax)

   added a commit that references this issue on Feb 14, 2019

   doing workaround based of microsoft/TypeScript#27413

   c28019a
6. fabb commented on Jan 1, 2020

   

   fabb

   on Jan 1, 2020

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   Maybe related:

   function getValues<T, K extends readonly (keyof T)[]>(object: T, keys: K): { [P in keyof K]: T[K[P]] } {
       return keys.map(key => object[key]) as any
   }
   

   This shows the error: Type 'K[P]' cannot be used to index type 'T'.
7. yseymour commented on Jun 6, 2020

   

   Yudhi Seymour (yseymour)

   on Jun 6, 2020

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   Closing this since it looks like the original issue is fixed.
8. Yudhi Seymour (yseymour)

   closed this as completedon Jun 6, 2020
9. microsoft

   locked as resolved and limited conversation to collaborators on Oct 21, 2025