Rest parameter in callback function using generic tuple types: Forces definition of all parameters

[BTOdell]

TypeScript Version: 3.8.0-dev.20191116

Search Terms:
rest parameter, spread, tuple type, generics, conditional types

Code
https://github.com/BTOdell/typescript-tuple-rest-spread-bug

This issue was discovered when implementing a type-safe event emitter class. I've produced a minimum reproducible example at the GitHub link above as well as in a Playground link below.

• index.ts defines an unimplemented EventEmitter that includes 4 functions which should be equivalent (except for the eventName parameter).
• test.ts consumes the EventEmitter class and calls the various functions to test type-safety.

An Events interface is used to define the supported events by mapping the event name to a tuple type for the parameters of the event listener.
This interface is passed through a generic parameter E in the Test class and then passed through a generic parameter E in the EventEmitter class.

Expected behavior:
A listener function should be able to define 1 parameter even if the caller might pass 2 parameters. The listener will then only have access to the first parameter passed. This is how JavaScript normally behaves.

Actual behavior:
The unexpected behavior occurs on lines 29 and 33 (in test.ts) and only occurs when the Events interface type has to pass through 2 generic class parameters.
If line 14 is uncommented, and line 13 is commented then all unexpected behavior is resolved.

Error:(29, 47) TS2345: Argument of type '(b: boolean) => void' is not assignable to parameter of type '(...args: Args<E["multiArray"]>) => any'.
  Types of parameters 'b' and 'args' are incompatible.
    Type 'Args<E["multiArray"]>' is not assignable to type '[boolean]'.
      Type 'E["multiArray"] | [E["multiArray"]]' is not assignable to type '[boolean]'.
        Type 'any[] & E["multiArray"]' is not assignable to type '[boolean]'.
          Types of property 'length' are incompatible.
            Type '2' is not assignable to type '1'.
              Type '[boolean, boolean]' is not assignable to type '[boolean]'.
                Types of property 'length' are incompatible.
                  Type '2' is not assignable to type '1'.

It seems like the tuple types [boolean, boolean] and [boolean] are not being treated like they're being spread as a rest parameter in a function. Could the context information be getting lost due to jumping through multiple generic parameters?

Playground Link:
Playground link

[jcalz]

Is this related to or a duplicate of #23132? Or maybe there's some other more canonical issue for discussing issues surrounding the compiler's inability/unwillingness to do much analysis on unresolved generic conditional types?

[BTOdell]

I read #23132 thoroughly and I believe it is related. I wouldn't call it a duplicate though. Ryan Cavanaugh said he was looking for concrete examples and I would say that this is one. I'm not doing anything overly complicated and unusual here... just trying to implement a fully type-safe event emitter.

But the issue definitely seems to be related to using constrained generic parameters along with conditional types.

[andrewbranch]

It’s not related to conditional types—here’s the same example without the conditional type, using an index signature to guarantee that E[keyof E] is an array type.

The use case here definitely seems reasonable... I think this is doable. No promises 😄