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1. mjbvz commented on Mar 29, 2022

   

   Matt Bierner (mjbvz)

   on Mar 29, 2022

   Last edited by mjbvz

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   This code previously worked for us in VS Code but is now causing around 11 errors in our codebase. Not sure if simply adding a extends object constraint is the correct fix in all cases
2. Matt Bierner (mjbvz)

   changed the title [-]Types no longer narrowed as expected [/-] [+]Generic type no longer narrowed as expected without `extends object`[/+] on Mar 29, 2022
3. whzx5byb commented on Mar 29, 2022

   

   whzx5byb

   on Mar 29, 2022

   Last edited by whzx5byb

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   It seems to be caused by #48366, which is tagged as "Breaking Change".

   Now you can't assign a unconstrained type parameter T to T extends {} or T extends object. See also #37388
4. DanielRosenwasser commented on Mar 29, 2022

   

   Daniel Rosenwasser (DanielRosenwasser)

   on Mar 29, 2022

   Last edited by DanielRosenwasser

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   CC Wesley Wigham (@weswigham)

   Looks like the following will work

   function deepEquals<T extends {} | null | undefined>(a: T, b: T): boolean {
       if (typeof a !== 'object' || typeof b !== 'object' || !a || !b) {
           return false;
       }
       if (Array.isArray(a) || Array.isArray(b)) {
           return false;
       }
       if (Object.keys(a).length !== Object.keys(b).length) {
           return false;
       }
       return true;
   } 

   and so will

   function deepEquals<T extends object | null | undefined>(a: T, b: T): boolean {
       if (typeof a !== 'object' || typeof b !== 'object' || !a || !b) {
           return false;
       }
       if (Array.isArray(a) || Array.isArray(b)) {
           return false;
       }
       if (Object.keys(a).length !== Object.keys(b).length) {
           return false;
       }
       return true;
   } 

   but not

   function deepEquals<T extends unknown>(a: T, b: T): boolean {
       if (typeof a !== 'object' || typeof b !== 'object' || !a || !b) {
           return false;
       }
       if (Array.isArray(a) || Array.isArray(b)) {
           return false;
       }
       if (Object.keys(a).length !== Object.keys(b).length) {
           return false;
       }
       return true;
   } 

   Note that the first two only likely work due to Anders Hejlsberg (@ahejlsberg)'s change in #43183. I don't know if we're willing to do something special for unknown or all non-unions.
5. Matt Bierner (mjbvz)

   added a commit that references this issue on Mar 29, 2022

   Pick up latest TS for building VS Code

   ...

   46abb2b
6. mjbvz commented on Mar 29, 2022

   

   Matt Bierner (mjbvz)

   on Mar 29, 2022

   Last edited by mjbvz

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   Also seeing this in the @playwright/test package. See this issue for repo steps and details about the error
7. Ryan Cavanaugh (RyanCavanaugh)

   added

   Breaking ChangeWould introduce errors in existing codeWould introduce errors in existing code

   on Mar 31, 2022
8. Ryan Cavanaugh (RyanCavanaugh)

   assigned

   Daniel Rosenwasser (DanielRosenwasser)

   on Mar 31, 2022
9. DanielRosenwasser commented on Apr 5, 2022

   

   Daniel Rosenwasser (DanielRosenwasser)

   on Apr 5, 2022

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   Playing around with a fix here FWIW

   #48576
10. TypeScript Bot (typescript-bot)

    added

    Fix AvailableA PR has been opened for this issueA PR has been opened for this issue

    on May 18, 2022
11. Anders Hejlsberg (ahejlsberg)

    closed this as completedin #49119on May 27, 2022
12. arnold-O commented on Feb 9, 2023

    

    Arnold Oghiator (arnold-O)

    on Feb 9, 2023

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    The Generic type without constraint is vague, hence with the extends keyword you clearly state what you want that Generic type to be, which is the reason for Typescript.