Design Meeting Notes, 1/24/2024

[DanielRosenwasser]

5.4 Breaking Changes

#57117

• Improve constraints of conditional types applied to constrained type variables #56004

  • Used to be that constraint of a conditional type on a type parameter would just run the constraint of the type variable through the conditional type.
    • But the concrete type could hold a more specific type, so that's incorrect.
  • Existing code assumes we will always evaluate using the based constraint.
  • In the error detector, this hits Drizzle, Qwik, react-hook-form, refined-github.
  • Particularly hard to figure out what is going on.
    • Are they all hard to fix?
    • Yes, sorta.
      • Qwik could rewrite a type to an intersection (we think).
      • react-hook-form could've made a type guard that's more specific to the codebase.

• Allow non-generic return types to be read from single generic call signatures #54477

  • What was the issue this was trying to fix?
  • No issue filed - but makes the behavior of couldContainTypeVariables observable (which is purely for performance).
  • Let's revert if possible.

• Reduce intersections of constrained type variables and primitive types #56515

  • Breaks a signature like <U, extends Equals<T, U>> in react-redux type helpers.
  • They can move the Equals out of the constraint, and rewrite it as an intersection.
    • <U extends T>(u: U & Equals<T, U>) => {}
  • Move Equals constraint into an intersection type. reduxjs/react-redux#2123

• Feelings?

  • Improve constraints of conditional types applied to constrained type variables #56004 is the hardest one to fix, but it is definitely more correct.
  • Will have to document it, we would like to keep that.

Supporting Named ESM Imports from the TypeScript API

#57133

• Proof-of-concept that adds an object that is friendly to cjs-module-lexer.
• If part of the build requires importing the compiler to build up that object, is that a security issue for PRs?
  • Already running build scripts etc.
• If you use export = , we emit stuff that is not friendly to cjs-module-lexer?
  • The lexer cannot deal with non-shorthand property declarations.
    • What???
• Feels like people are not happy with it since we don't fully understand the implications, but not strongly against it.