Incorrect overload is used for pipe when first function has optional parameter #29913

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1. RyanCavanaugh commented on Feb 27, 2019

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Feb 27, 2019

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   The first overload that matches is always chosen. The pipe overloads should be in the other order.
2. Ryan Cavanaugh (RyanCavanaugh)

   added

   Working as IntendedThe behavior described is the intended behavior; this is not a bugThe behavior described is the intended behavior; this is not a bug

   on Feb 27, 2019
3. OliverJAsh commented on Feb 27, 2019

   

   Oliver Joseph Ash (OliverJAsh)

   on Feb 27, 2019

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   How come this behaviour only applies when strictFunctionTypes is disabled?

   This is the order of overloads used by Lodash's flow (aka pipe) function. I have raised a PR to fix the overload order, as you suggested: DefinitelyTyped/DefinitelyTyped#33450.
4. RyanCavanaugh commented on Feb 27, 2019

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Feb 27, 2019

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   strictFunctionTypes off means that parameter types are compared bivariantly, so the {} | undefined being assignable from the implicit no-argument undefined is acceptable.
5. Oliver Joseph Ash (OliverJAsh)

   closed this as completedon Feb 28, 2019
6. OliverJAsh commented on Sep 18, 2019

   

   Oliver Joseph Ash (OliverJAsh)

   on Sep 18, 2019

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   The first overload that matches is always chosen. The pipe overloads should be in the other order.

   If we do this, it fixes the example above, but it causes other issues:

   {
       declare const pipe: {
           // 1-argument first function
           <A, B>(ab: (a: A) => B): (a: A) => B;
   
           // 0-argument first function
           // Workaround: disable this overload
           <A>(a: () => A): () => A;
       };
   
       // Expected and actual type: `(a: {} | undefined) => number`
       const fn1 = pipe((_a?: {}) => 1);
   
       // Expected type: () => number
       // Actual type: (a: unknown) => number
       const fn2 = pipe(() => 1);
   }

   Workaround: use generic rest parameters:

   {
       declare const pipe: {
           <A extends any[], B>(ab: (...a: A) => B): (...a: A) => B;
       };
   
       // Expected and actual type: `(a: {} | undefined) => number`
       const fn1 = pipe((_a?: {}) => 1);
   
       // Expected and actual type: () => number
       const fn2 = pipe(() => 1);
   }
7. microsoft

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