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

Start with the linked TypeScript Playground reproduction and compare its one-level conditional behavior with the approach described in pull request 56004. Trace how the nested conditional type in the example is evaluated for constrained type variables, then verify that the fourth assignment produces an error while preserving the reported behavior for the other assignments.