Constraints of nested conditional types applied to constrained type variables are incorrect

🔎 Search Terms

constraints conditional types type variables nested

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.0-dev.20240904#code/C4TwDgpgBAkgzgWQIbAMYAsCWA7A5gZWACcdcB5bCAGQgDcIAbAHgBUoIAPYCbAEzihxipAHxQAvFDadufAQAMkAEgDeQkngC+8gFBQoAfijEArhD1QAXFABmSBnAgBuHTpsnsqYJgD22WwBMTPjsXDz8gsJ4IgAUHNbwyGhYeIQa5JQ09Mz4IgCUUCoWDBDAxtam0JIcTlAA9HVQAKJERD5ExaW21nYOVVA19Y0tbR2arqCQsIgoGKRppCwA7j5ZjHCsobIR6qISUlvhCspqUbjaFkbSYXJQ8qq7WgBeuvr6RpUW+j32jhY-fRcbg8Xl8-hsAGZgodbo9cLF4tMknNUmdlqs6OtgvlCp0ysAKkQzPtBg1mq12njurZfv1SY0yABrKAAIxMZTg6B8JgYvFZ0CQ-ggFLGOiAA

💻 Code

type IsMatchingStringOneLevel<T extends string> = T extends `a${string}`
  ? true
  : false;

function f2<S extends string>(x: IsMatchingStringOneLevel<S>) {
  let t: true = x; // Error
  let f: false = x; // Error
}

type IsMatchingStringTwoLevels<T extends string> = T extends `a${string}`
  ? T extends `${string}z`
    ? true
    : false
  : false;

function f3<S extends string>(x: IsMatchingStringTwoLevels<S>) {
  let t: true = x; // Error
  let f: false = x; // Ok but should be an error
}

🙁 Actual behavior

It doesn't error on the fourth assignment

🙂 Expected behavior

It should error

Additional information about the issue

This is just a variant of what was fixed for one-level conditionals in https://github.com/microsoft/TypeScript/pull/56004

Inizia con la riproduzione collegata in TypeScript Playground e confronta il suo comportamento condizionale a un livello con l’approccio descritto in pull request 56004. Traccia come viene valutato il tipo condizionale annidato nell’esempio per le variabili di tipo vincolate, quindi verifica che la quarta assegnazione produca un errore, preservando il comportamento riportato per le altre assegnazioni.