Open Radar

 
Summary:

Swift's struct initialisers look like functions but don't behave as such.

Example:

Given struct S and a function sConstructorFun

struct S {
    let a : String 
    let b : Int 
    let c : Bool 
}

func sConstructorFun(a a:String, b b:Int, c c:Bool) -> S { 
    return S(a:a, b:b, c:c) 
}

I can use both sConstructorFun(a:"", b:1, c:false) and S(a:"", b:1, c:false) to get the following S value (as the REPL outputs it)

S = {
  a = ""
  b = 1
  c = false
}

So, S and sConstructorFun have the very same interface and unsurprisingly return the same result.

However, a sFactory function defined as follows

func sFactory(f:(String, Int, Bool) -> S) -> S {
    return f("foo", 42, false) 
}

can only be used with the sConstructorFun but not with S directly:

REPL> sFactory(sConstructorFun) 
$R2: S = {
  a = "foo"
  b = 42
  c = false
}

but

REPL> sFactory(S) 
repl.swift:18:1: error: cannot invoke 'sFactory' with no arguments
sFactory(S)
^
repl.swift:18:9: note: expected an argument list of type '((String, Int, Bool) -> S)'
 sFactory(S)
         ^


Steps to Reproduce:
- Save the attached file main.swift
- Run `xcrun swift main.swift`

Expected Results:
compiles all fine

Actual Results:
main.swift:18:9: error: cannot invoke 'sFactory' with no arguments
let d = sFactory(S) /* compile error */
        ^
main.swift:18:17: note: expected an argument list of type '((String, Int, Bool) -> S)'
let d = sFactory(S) /* compile error */
                ^

Regression:
no