Crash on 5.4.0-dev.20231206 #56688

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1. Andarist commented on Dec 6, 2023

   

   Mateusz Burzyński (Andarist)

   on Dec 6, 2023

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   Does it crash on 5.4.0-dev.20231206 too?
2. ArnaudBarre commented on Dec 6, 2023

   

   Arnaud Barré (ArnaudBarre)

   on Dec 6, 2023

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   It crashes on 20231206 and 20231205, not on 5.3.2. Didn't try other versions
3. Andarist commented on Dec 6, 2023

   

   Mateusz Burzyński (Andarist)

   on Dec 6, 2023

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   removeConstrainedTypeVariables was introduced in #56515 . Could you confirm that 5.4.0-dev.20231130 works OK but 5.4.0-dev.20231201 is broken? Even with that confirmation - it might be hard to fix this without a repro case. If you'd be interested I could try to guide you on how to at least narrow it down to the file in which the problem occurs. With that information, you'd have an easier time dissecting this to a minimal repro case.
4. ArnaudBarre commented on Dec 7, 2023

   

   Arnaud Barré (ArnaudBarre)

   on Dec 7, 2023

   Last edited by ArnaudBarre

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   Ok so yes it crashes starting on 20231201.
   I ran the debugger and found the file, and after few try and errors I think this is this combination:

   // .d.ts
   declare global {
     interface String {
       upperFirst<T extends string>(this: T): Capitalize<T>;
     }
   }
   
   // display.ts
   const displayEnum = (value: string) => value.toLowerCase().upperFirst();
   
   // Component.tsx
   <>{displayEnum("VALUE")}</> 
   
5. jakebailey commented on Dec 8, 2023

   

   Jake Bailey (jakebailey)

   on Dec 8, 2023

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   It's in the template, but you can also use https://www.npmjs.com/package/every-ts to narrow it down further.

   I can't get your case to crash in the test suite, unfortunately.
6. ArnaudBarre commented on Dec 8, 2023

   

   Arnaud Barré (ArnaudBarre)

   on Dec 8, 2023

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   Ok this is the missing piece, the span before is strangely required:
   Here is my mininal repro, I don't think my TS config should have an impact on the crash

   declare global {
     interface String {
       upperFirst<T extends string>(this: T): Capitalize<T>;
     }
   }
   
   const displayEnum = (value: string) => value.toLowerCase().upperFirst();
   
   console.log(
     <div>
       <span>Scope:</span> {displayEnum("VALUE")}
     </div>,
   );
   
   export {};
   
7. Ryan Cavanaugh (RyanCavanaugh)

   added

   BugA bug in TypeScriptA bug in TypeScript

   on Dec 8, 2023
8. Ryan Cavanaugh (RyanCavanaugh)

   added this to the Backlog milestone on Dec 8, 2023
9. Jake Bailey (jakebailey)

   closed this as completedin #56711on Dec 11, 2023
10. microsoft

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