Design Meeting Notes, 5/8/2020

[RyanCavanaugh]

• PR Consistent narrowing by discriminant #38311 More-consistent narrowing
  • Fixes long-standing issue
  • Design limitation was due to some caching issues
  • Singleton types still won't narrow to never, which is generally the desired behvioar
• Named tuples
  • Switching to parameter-style syntax based on poll feedback
  • All names must be present or not present
  • Names are preserved through most transforms
  • Good to go for 4.0
• static index signatures on classes
  • ASI issues in parsing
  • Final approval 👍
• Exponential increase in compile time and memory usage #38339 performance issues
  • Caused by massive unions (JSX.IntrisicElements[keyof JSX.IntrinsicElements]) going through subtype reduction
  • Keep investigating

[DanielRosenwasser]

I ended up taking notes as well 😬

More Consistent Narrowing

#30557

• Unless the declared type is a union, we don't do discriminant narrowing.
• In a lot of cases, people start out with a declared type of unknown, guard down to a discriminated union, and then try to do a discriminant check.
  • Users are often very surprised by this.
• Idea: if the declared type is a union, use that. If not, use the currently-narrowed type.
• Why can't we always use the computed type?
  • The problem is that if you had a singleton value type - do you really want to narrow that down to the never case?

Labeled Tuples

#28259

• Got language service support working
  • Doc comments seem to be tracked around
• Based on polls, we'll have to swap the syntax a little bit.
• What about mixing unlabeled and labeled?
  • Not possible.
  • "Gracefully handled" by giving them no name for any subsequent analysis.

Time Blowup

#38339

• Spending time in subtype reduction on massive types that are...actually the same type!
• But even if they weren't the same types, this takes a long time!
• Subtype reduction - is it worth it? Really, these types should be the same!
  • Also, for a union of 100 members...maybe who cares!? The point is usually to make the unions are small.

[AviVahl]

Not sure if I'm the first one to suggest it... Any chance you guys could video record and upload these meetings? :) (like Node.js does)

Would be really nice to be "a fly on the wall".

EDIT: heck, it's 2020, make a live stream.