Design Meeting Notes, 8/17/2018

[RyanCavanaugh]

• Generic inference inside of pipe is incorrect when strict mode is disabled #25826 Inference of pipe-like functions
  • No obvious solution that doesn't break other stuff
  • The only real fix would be full unification
• keyof and type parameters can produce huge types that hang/crash compiler #25942 OOM crash or non-ending computations
  • Can we limit ourselves in some way to prevent OOMs?
  • We could make createType fail, but it's not clear what to do afterwards
  • The type here is not particularly deep, just wide
    • No obvious problem is detectable during checking
  • See also [Bug]tsc does not finish #26314
  • What useful thing could we do if we detected we were about to OOM?
    • Throw?
    • Try to "stop" checking?
    • Throw a special exception to disable the LS
  • What about hangs?
  • Would need VS / VS Code to support some kind of detection
  • Keep trying to find a solution
• Partial type argument specification and named type arguments (Implement partial type argument inference using the _ sigil #26349)
  • How does the call site specify the inference positions?
    • Difficulty is that default type parameters exist
    • We need some placeholder
    • Big breaking change to switch nonspecified parameters to inference
  • Presume you have Foo<A, B, C = string>, then Foo<T, U> is different from Foo<T, U, >
    • Gross
  • Not much consensus for * or auto or infer
  • What about inferring only when no default exists?
    • More subtlety
  • OK maybe * is the best
  • What's the use case?
    • Vue
    • Why aren't Vue's type parameters in the "right" order?
      • One person made a mistake and now we're stuck with it!
  • 👍 for *
  • Semantics are good to go
• Fallout from Narrowing on assignment does not remove weak types #26405 Narrowing mapped types
  • "Is maybe assignable to" is almost the same as "Is comparable to"
  • Break in RWC in T | T[] with T = { x?: string }
    • Kinda wat because T[] is also T for weak T
  • Control flow is based on removal of constituents of unions
  • It's "incorrect" to remove T from T | T[] in the AOrArr<T> example
  • Remember that weak types are comparable to but not assignable to most target types
  • Would we rather have Generic lookup type wrongly narrowed to never with strictNullChecks #26130 or Narrowing on assignment does not remove weak types #26405?
    • Generic lookup type wrongly narrowed to never with strictNullChecks #26130 - not a regression, but weird
  • T | T[] with weak T is suspect but not uncommon
  • (Debate about why Generic lookup type wrongly narrowed to never with strictNullChecks #26130 happens)
  • Don't add a new type relation yet - high perf cost and new subtleties
  • Roll back the fix for now and try to think of other fixes
• And expression inferrs wrong type. #26468 && in non-SNC
  • The rules are ancient and also bad
  • Right operand is a good answer for non-primitive LHSes in non-SNC but bad for primitives
  • Fun breaking change
  • Isn't all of this code extremely suspect?
  • See what's in RWC
  • Everyone should turn on SNC!
  • No urgency
• Distribute indexed accesses when simplifying them #26281 for next week along with Nathan's inference-based refactors