Design Meeting Notes, 9/3/2021

[DanielRosenwasser]

Class Define Semantics and Decorators (useDefineForClassFields + experimentalDecorators)

#45653 (comment)

• Recently turned on useDefineForClassFields in TS4.4 + target: esnext.
• Broke people using decorators.
  • One example given replaces a property with a getter/setter pair.
• No way to provide a heads up that it's not going to work.
• Could add a lint rule.
  • Nobody's going to see that probably.
• Maybe we should provide a suggestion or error on experimentalDecorators and esnext?
  • Don't want to add a new flag to say "I know what I'm doing"
  • experimentalDecorators: "iknowhwatimdoing"
• The workaround today is to add the declare operator on the property.
  • But we don't have a way to message it.
• Maybe should just continue to keep it the same.
• What if a decorated thing, when using experimentalDecorators opted you out of Define semantics?
  • "That sounds horrifying to me"
• Feels like we're okay with a suggestion diagnostic in tsconfig.json.
  • Does it have a quick fix?
  • Doesn't have to.
  • Could make it turn useDefineForClassFields off.
  • Not sure if this is really visible either.

Optimizing on Tail Recursive Conditional Types

#45711

• Have to enforce some limit because it's possible to write never-ending recursive types.
  • Currently at 1000 iterations.
• Lots of non-tail-recursive types, so you have to create an accumulator.
• No breakages we introduce from what we see - but do see a slight regression on material-ui.
• What happens if someone regresses you to non-tail-call?
  • Some languages allow you to specify whether you need a function to be tail-call optimized.
  • It's really the branch, not the type that is optimizable.
• Shouldn't force everyone to have an annotation - engines did this.
  • Different for engines. Debugging type instantiation (while nice) is not nearly as much a scenario as debugging your tail-recursive functions.
• Could always introduce the branch marker after.
• Any way to turn it off?
  • Can't imagine why you would want that (famous last words).
  • We have a lot of users with interesting use-cases. 😅
  • TS will do more work for longer without just hitting an error.
  • You can always union in each branch with a sentinel type and then strip it out after.

CLI Improvements

• Error summary? Listing files and error count.
  • Tools will have to adjust their regexes.
• tsc --init
  • Show the actual options and clean up the errors.
  • Should it leverage --showConfig?
  • What do we do when a LOT of options get printed?
  • Could show only the diff from the defaults.
  • Still a lot.
• How we present multiple lines
  • Could do multiple versions depending on the type of error.