Function parameter type inference error #59949

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1. MartinJohns commented on Sep 12, 2024

   

   Martin Johns (MartinJohns)

   on Sep 12, 2024

   Contributor

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   Because checking what value type is does not have any relevance for what type the value of extend is.

   This is a perfectly valid call: func<Type.A | Type.B>(Type.A, 1);
2. lengyuxuan commented on Sep 12, 2024

   

   fangjin (lengyuxuan)

   on Sep 12, 2024

   Author

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   Because checking what value type is does not have any relevance for what type the value of extend is.

   This is a perfectly valid call: func<Type.A | Type.B>(Type.A, 1);

   How should I constrain the type of "extend" ? Do you have any suggestions ? Thank you so much !
3. jcalz commented on Sep 12, 2024

   

   Joe Calzaretta (jcalz)

   on Sep 12, 2024

   Contributor

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   Duplicate #33014

   For now, if you need this behavior you either need to rewrite in terms of #47109 and use generic indexing instead of control flow anlaysis:

   function func<T extends Type>(type: T, extend: X<T>) {
       const x: { [K in Type]: (type: K, x: X<K>) => void } = {
           [Type.A](type, extend) {
               console.log(type, extend);
               //                ^? (parameter) extend: string
           },
           [Type.B](type, extend) {
   
           }
       }
       x[type](type, extend);
   }
   func(Type.A, '');
   func(Type.B, 1);
   func(Math.random() < 0.99 ? Type.A : Type.B, 1); // still allowed, #47109 technically unsound this way

   or, you need to use control flow analysis on a discriminated union and not use generics:

   function func(...[type, extend]: { [T in Type]: [type: T, extend: X<T>] }[Type]) {
       if (type === Type.A) {
           console.log(type, extend);
           //                ^? (parameter) extend: string
       }
   }
   
   func(Type.A, '');
   func(Type.B, 1);
   func(Math.random() < 0.99 ? Type.A : Type.B, 1); // error

   Playground link to code

   👍React with 👍2Reacted by Ryan Cavanaugh and fangjin
4. lengyuxuan commented on Sep 13, 2024

   

   fangjin (lengyuxuan)

   on Sep 13, 2024

   Author

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   Duplicate #33014

   For now, if you need this behavior you either need to rewrite in terms of #47109 and use generic indexing instead of control flow anlaysis:

   function func<T extends Type>(type: T, extend: X<T>) {
       const x: { [K in Type]: (type: K, x: X<K>) => void } = {
           [Type.A](type, extend) {
               console.log(type, extend);
               //                ^? (parameter) extend: string
           },
           [Type.B](type, extend) {
   
           }
       }
       x[type](type, extend);
   }
   func(Type.A, '');
   func(Type.B, 1);
   func(Math.random() < 0.99 ? Type.A : Type.B, 1); // still allowed, #47109 technically unsound this way

   or, you need to use control flow analysis on a discriminated union and not use generics:

   function func(...[type, extend]: { [T in Type]: [type: T, extend: X<T>] }[Type]) {
       if (type === Type.A) {
           console.log(type, extend);
           //                ^? (parameter) extend: string
       }
   }
   
   func(Type.A, '');
   func(Type.B, 1);
   func(Math.random() < 0.99 ? Type.A : Type.B, 1); // error

   Playground link to code

   Good job
5. fangjin (lengyuxuan)

   closed this as completedon Sep 13, 2024
6. fangjin (lengyuxuan)

   reopened this on Sep 13, 2024
7. fangjin (lengyuxuan)

   closed this as completedon Sep 13, 2024
8. microsoft

   locked as resolved and limited conversation to collaborators on Oct 22, 2025