Design Meeting Notes, 3/6/2020

[DanielRosenwasser]

Assignability Between {} and Unconstrained Type Parameters

#33570

• When we added unknown, we also added a special case for unknown to be assignable to {} and Object.
• A while back, we changed the default type parameter constraint of {} to unknown
• This actually broke us as well!
  • A couple of places where we mixed and matched between {} and unknown.
• Lots of failures, but mostly failures in the same package.
  • schema<T> where T really should've been updated to {}.
• Does this break type parameters that explicitly extend {}?
  • No, there's no special casing for type parameters.
• Is it pretty easy to fix?
  • Pretty consistent that people just have to change the constraints to only accept {} or object.
• Not such a big fan of more breaks in the same release.
  • But the error messages are consistent around {}.
• What about when you're not in strict mode?
  • There, unknown and {} are really the same thing.
  • Maybe that's why we kept the rule in the first place.
  • unknown is already assignable to {} outside of strictNullChecks, so this doesn't affect users there.
• Who'd it break?
  • DefinitelyTyped
  • Office UI Fabric
    • Seems to be from DefinitelyTyped changes
• Conclusion
  • Try to see whether we can at least give an actionable error on this change.
  • Let's hold it off for another release.

Update on Intersection Assignability with Optional Properties

#37195

• Office UI Fabric got hit, gave them an update.

Checking Index Signatures In the Presence of Optional Properties

#27591

• https://github.com/mattmccutchen/TypeScript/pull/1/files#diff-601ae163618e1a582913605ac0b1b7c1R104-R116

• https://github.com/mattmccutchen/TypeScript/pull/1/files#diff-0e62d568d28b82cee9e66b2b8fafd290R7-R42

• Is debatable - doesn't mean that all the properties are there

  • Depends on the intent

• Definitely catches bad code!

  • Breaks VS Code and Azure, but these are correct breaks!

• Delay this for the next release.

• Lets give known broken partner teams a heads up

  • @weswigham to tarball this
  • @sandersn reach out to VS Code
  • @DanielRosenwasser reach to Azure SDK