Suggestion Backlog Slog, 1/29/2018

[RyanCavanaugh]

• Private properties should still emit type #20979 Classes using only private consumption of generic variables don't have correct .d.ts emit
• suggestion: correct types of flag booleans for RegExp literals #21290 Literalize the flags of RegExp literals
• Enum literal cannot contain concatenated string #20784 Support basic concat in string enum values
• += on string | undefined should narrow its operand to string #20203 += should remove undefined from a string | undefined value
• Possible to narrow type of object literal property with strictNullChecks? #20219 Extant properties in object literals should be treated as assignments for flow control analysis purposes
• Allow static assertions #18523 Block-level asserts would still be nice
• strictNullChecks blocks safe typeof capture #16108 Remove undefined from domain of x in typeof x.y
• Ability to group errors together #15550 Group errors with same text
• Request: Allow abstract classes to implement MappedTypes that are instantiated with type parameters. #21326 Allow an (abstract?) class to implement mapped types
• Unable to destructure TypedArrays #20623 Allow destructuring of typed arrays
• Jquery error TS1209: Ambient const enums are not allowed when the '--isolatedModules' flag is provided. #20703 Allow declararation (not use) of const enums in --isolatedModules
• Strict function check failure due to union type with matching parameters in the unioned interfaces #20714 Be more selective in inference between union types by attempting to pairwise match types in the source and target to avoid "cross contamination"
• Allow type assertions from T-constrained type variable to subtypes of T #20821 Allow assertions from U-constrainted type parameter T to subtypes of U
• narrowing in switches doesnt work on constrained unions #20375 Don't produce never when a discriminated union from a constraint is exhausted
• Need way to express hybrid types that are indexable for a subset of properties  #17867 Status check on "leftover" index signatures
• Assignments in the try-block are unconditionally assumed to have never happened in the catch-block #16682 In catch block, CFA wrongly assumes assignments from try block must have never occurred

[RyanCavanaugh]

• Private properties should still emit type #20979 Classes using only private consumption of generic variables don't have correct .d.ts emit

  • We need (?) to write out the type, somehow
  • But we don't actually know if the type parameter is in the property type
  • The type of the private property may not be speakable in the .d.ts file
    • Would be a breaking change to enforce speakability of private properties' types
  • We need to know how it's actually used to correctly determine variance
  • Only affects instances where you'd pass one Gen<T> to another Gen<U>
  • No clear fix
    • Possible hack: Emit the type parameter if it's exactly T (or some other type parameter)
  • Workaround is to declare a psuedoproperty
  • This bug has existed foreverish
  • Won't Fix

• suggestion: correct types of flag booleans for RegExp literals #21290 Literalize the flags of RegExp literals

  • Seems like an odd corner case to care about
  • You could write a function sticky(rgx: string) { return new RegExp(rgx, 's') }
  • 👎

• Enum literal cannot contain concatenated string #20784 Support basic concat in string enum values

  • 👍
  • Accepting PRs

• += on string | undefined should narrow its operand to string #20203 += should remove undefined from a string | undefined value

  • Add a flag?
  • Sidebar on concating string with non-strings & implicit conversions
    • Flaaaaags
  • Should we just disallow concat with undefined / null ?
    • Sidebar on implicit concat of string and number
  • string | undefined + string | undefined currently produces string but is actually NaN at runtime when both operands are undefined
  • Under strictNullChecks as a breaking change? With no opt-out?
  • JSON.stringify does return undefined when passed a function object
    • But is currently typed incorrectly as returning string
  • Flaaaaaaaaaaaaags
  • Ryan would rather get an error when concating function or object than null or undefined
  • Tried to force Mohamed to have an opinion but nothing happens
  • No agreement on what constitutes "better" relative to current behavior / breaking change constraints
  • Original issue is still open

• Possible to narrow type of object literal property with strictNullChecks? #20219 Extant properties in object literals should be treated as assignments for flow control analysis purposes

  • Discussion of implementation difficulty
    • "Doable but not trivial"
  • Restrict to bare initializations of the variable
  • Committed but no promises
  • Contingent on difficulty

• Allow static assertions #18523 Block-level asserts would still be nice

  • The statement-form assertion has easily-understood semantics
  • Haven't found a good color for the bikeshed
  • All CFA is contingent on bind-time information
  • May or may not be bound to function calls
  • Meditate and hope for syntactic inspiration

• strictNullChecks blocks safe typeof capture #16108 Remove undefined from domain of x in typeof x.y

  • We talked about this at a prior design meeting but no one can remember why we didn't want to do this
  • 👍
  • Accepting PRs

• Request: Allow abstract classes to implement MappedTypes that are instantiated with type parameters. #21326 Allow an (abstract?) class to implement mapped types

  • It's really like lateimplements ?
  • Much easier to validate this constraint in a call position of an instance
    • Like static implements
  • Do these objects ever get passed anywhere? Why not validate at a call site?
  • AMF

• Unable to destructure TypedArrays #20623 Allow destructuring of typed arrays

  • Why doesn't it just work already?
  • If it just works let's just go allow it
  • Assign to Ron
  • Sidebar: Complaining about Playground

• Jquery error TS1209: Ambient const enums are not allowed when the '--isolatedModules' flag is provided. #20703 Allow declararation (not use) of const enums in --isolatedModules

  • Mohamed says .d.ts files shouldn't have const enums
    • Yes, ban them
  • Also change logic to only error when the enum is referenced
  • 👍

• Allow type assertions from T-constrained type variable to subtypes of T #20821 Allow assertions from U-constrainted type parameter T to subtypes of U

  • This error doesn't help anyone
  • Allow cross-hierarchy comparisons for comparability relation?
  • No pure principle of reasoning to disallow certain things but not others
    • Any T might be a U because T & U exists
  • 👍 to adding this to the realm of plausability
  • Assign to Daniel to see if he can spec it

• narrowing in switches doesnt work on constrained unions #20375 Don't produce never when a discriminated union from a constraint is exhausted (correction: do the complete opposite of that)

  • Bug

• Assignments in the try-block are unconditionally assumed to have never happened in the catch-block #16682 In catch block, CFA wrongly assumes assignments from try block must have never occurred

  • Use the same control flow graph for the catch block as for "after the try block"
  • No mechanisms in place to capture "all possible exits due to exceptions" in a control graph
  • Too Complex

• Out of time

  • Strict function check failure due to union type with matching parameters in the unioned interfaces #20714 Be more selective in inference between union types by attempting to pairwise match types in the source and target to avoid "cross contamination"
  • Need way to express hybrid types that are indexable for a subset of properties  #17867 Status check on "leftover" index signatures
  • Ability to group errors together #15550 Group errors with same text