Unexpected error/bad inference for generic composed function inside pipe #25791

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1. Mohamed Hegazy (mhegazy)

   assigned

   Wesley Wigham (weswigham)

   on Jul 19, 2018
2. Mohamed Hegazy (mhegazy)

   added

   Needs InvestigationThis issue needs a team member to investigate its status.This issue needs a team member to investigate its status.

   on Jul 19, 2018
3. OliverJAsh commented on Feb 13, 2019

   

   Oliver Joseph Ash (OliverJAsh)

   on Feb 13, 2019

   ContributorAuthor

   More actions

   Revisiting this. I believe the issue can be narrowed down to:

   {
       declare const pipe: {
           <A, B, C, D>(ab: (a: A) => B, bc: (b: B) => C, cd: (c: C) => D): (a: A) => D;
       };
   
       declare const getArray: () => string[];
       declare const first: <T>(ts: T[]) => T;
   
       /*
       Without `strictFunctionTypes` enabled:
       Expected type: `(a: {}) => string`
       Actual type: `(a: {}) => {}`
       */
       const fn = pipe(
           getArray,
           x => x,
           /*
           With `strictFunctionTypes` enabled, unexpected type error:
           Argument of type '<T>(ts: T[]) => T' is not assignable to parameter of type '(c: {}) => {}'.
               Types of parameters 'ts' and 'c' are incompatible.
                   Type '{}' is missing the following properties from type '{}[]': length, pop, push, concat, and 25 more.
           */
           first,
       );
   
       // Workaround 1: use `identity` function
       declare const identity: <T>(value: T) => T;
       const fn2 = pipe(
           getArray,
           identity,
           first,
       );
   
       // Workaround 2: wrap last function
       const fn3 = pipe(
           getArray,
           x => x,
           x => first(x),
       );
   }

   Any ideas Mohamed Hegazy (@mhegazy) Wesley Wigham (@weswigham)? Does this help?
4. Oliver Joseph Ash (OliverJAsh)

   changed the title [-]Unexpected error/bad inference for generic inside of index signature[/-] [+]Unexpected error/bad inference for generic composed function inside `pipe`[/+] on Feb 14, 2019
5. Anders Hejlsberg (ahejlsberg)

   closed this as completedin #30193on Mar 8, 2019