The behavior is different for two code that looks the same #30988

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1. jack-williams commented on Apr 17, 2019

   

   Jack Williams (jack-williams)

   on Apr 17, 2019

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   I think the effect of the type alias F is causing reduction in the body of the alias, in particular, I think it's simplifying the return type to the constraint of the conditional 1 | 2. Once this happens, the type parameter X no longer appears in the type and the two signatures are related.

   type F<X> = <T>() => T extends X ? 1 : 2;
   declare const x: F<number>;
   const y: F<string> = x; // no error
   
   type F2<X> = <T>() => T extends X ? 1 : 2;
   const y2: F2<string> = x; // error

   Haven't got as far as tracking down where exactly this behaviour comes from, but I'm guessing there is some instantiation business going on.
2. Ryan Cavanaugh (RyanCavanaugh)

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

   on Apr 17, 2019
3. kgtkr commented on Apr 18, 2019

   

   kgtkr (kgtkr)

   on Apr 18, 2019

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   Related code

   
4. Ryan Cavanaugh (RyanCavanaugh)

   added

   UnactionableThere isn't something we can do with this issueThere isn't something we can do with this issue

   and removed

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

   on Aug 22, 2019
5. RyanCavanaugh commented on Aug 22, 2019

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Aug 22, 2019

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   These samples are quite different and it's not apparent why they must behave identically.
6. Ryan Cavanaugh (RyanCavanaugh)

   closed this as completedon Aug 22, 2019