Design Meeting Notes, 2/2/2024

[DanielRosenwasser]

Order-Dependency with Contextual Typing and Discriminated Unions

#57231

• Discriminated unions are really unions of objects that have discriminant properties.
• We say that a property is considered a discriminant if its type differs in at least two constituents, and one of those constituents is a literal-like type.
• For template string types, you need information about the contextual type before you can make any decision about the template literal itself.
  • So we can't get a narrowed contextual type.
  • End up not getting a more specific type for other properties, assignment fails.
• In general, overlapping discriminant types is a bad idea.
• A few options.
  • Do nothing.
  • Don't consider properties discriminants if one type subsumes the other.
    • Doesn't work.
  • Create a notion of "better" discriminants, discriminate on those first.
• Why is it the case that we don't create literal types out of template string expressions wihtout a contextual types?
  • It was too breaky.