[SR-8797] Misleading compiler errors

[jeremyabannister]

Previous ID	SR-8797
Radar	rdar://problem/44616337
Original Reporter	@jeremyabannister
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, DiagnosticsQoI, StarterBug, TypeChecker
Assignee	None
Priority	Medium

md5: f2dc6c9ca2ac0b74c004d15fc50e5a7b

Issue Description:

typealias Executable <Input> = (Input)->()
let test: Executable<Void>? = { }

The code above leads to the error:

Cannot convert value of type '() -> ()' to specified type '((Void) -> ())?'

Whereas this code compiles successfully:

typealias Executable <Input> = (Input)->()
let test: Executable<Void>? = { _ in }

[belkadan]

That sounds correct to me. A function that takes one argument is different from a function that takes zero arguments, even when that one argument is Void (the empty tuple). What diagnostic would you have preferred?

[belkadan]

cc @rudkx

[rudkx]

Yes this is expected behavior at this point.

I floated a pitch at one point to fix this because it makes it difficult to write generic code that composes well in some cases, but there was little to no enthusiasm for doing anything about it and I haven't had time to pursue it since.

[rudkx]

@xedin, perhaps we could have a better diagnostic here to make it more clear what the issue is?

[xedin]

Yes, I think it makes sense to say that it expected on argument not got 0 or something similar...

[xedin]

@swift-ci create

[jeremyabannister]

@belkadan @rudkx @xedin

Thanks for the speedy responses guys - I realize now that the error is perfectly fine, this was just a gap in my understanding of `Void`. What I wish is that there were a way to use my `Executable<Input>` typealias to represent `()->()`. Given the fact that there is only one possible value of `Void` it does feel to me that the need for that input qualifies as boilerplate code, but I can understand that introducing the idea of treating a function type with no arguments as being equivalent to a function type with one (trivial) argument might be very unsound.