[SR-11778] Callable types do not respect default argument values

[xedin]

Previous ID	SR-11778
Radar	rdar://problem/56998726
Original Reporter	@xedin
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	@hamishknight
Priority	Medium

md5: db8faab81e6ef687c92bbc71a7ec459b

Issue Description:

Default argument values are not respected when using the newly added callAsFunction/static callable feature of Swift 5.1. See example playground. I would expect the code to execute without an error, but instead the compiler wants me to provide a value for “completion:” even though I define a default value in the callAsFunction method.

struct DoubleANumber {
  func callAsFunction(_ x: Int, completion: (Int) -> Void = { _ in }) {
    completion(x + x)
  }
}

let x = DoubleANumber()
x(3)

[dan-zheng]

@xedin: do you have any tips for debugging this kind of issue?

I only know -Xfrontend -debug-constraints, but I'm not sure how to interpret the output:

$ swift -Xfrontend -debug-constraints sr-11778.swift
...
---Solver statistics---
Total number of scopes explored: 6
Maximum depth reached while exploring solutions: 3
Time: 2.384000e+00ms
---Solution---
Fixed score: 2 0 0 0 0 0 0 0 0 0 0 0
Type variables:
  $T4 as (Int) -> Void @ locator@0x7ff0e7002d58 [Call@sr-11778.swift:8:1 -> apply argument -> comparing call argument #​1 to parameter #​1 -> synthesized argument #​1]
  $T3 as (Int, (Int) -> Void) -> () @ locator@0x7ff0e7002c28 [Call@sr-11778.swift:8:1 -> member]
  $T2 as () @ locator@0x7ff0e7002b78 [Call@sr-11778.swift:8:1 -> function result]
  $T0 as DoubleANumber @ locator@0x7ff0e7002a00 [DeclRef@sr-11778.swift:8:1]
  $T1 as Int @ locator@0x7ff0e7002a98 [IntegerLiteral@sr-11778.swift:8:3]

Overload choices:
  locator@0x7ff0e7002c08 [Call@sr-11778.swift:8:1 -> apply function] with main.(file).DoubleANumber.callAsFunction(_:completion:)@sr-11778.swift:2:8 as DoubleANumber.callAsFunction: (Int, (Int) -> Void) -> ()

  locator@0x7ff0e7002a00 [DeclRef@sr-11778.swift:8:1] with main.(file).x@sr-11778.swift:7:5 as x: DoubleANumber


Constraint restrictions:

Disjunction choices:

Fixes:
  [fix: synthesize missing argument(s)] @ locator@0x7ff0e7002d38 [Call@sr-11778.swift:8:1 -> apply argument]
sr-11778.swift:8:4: error: missing argument for parameter 'completion' in call
x(3)
   ^
   , completion: <#(Int) -> Void#>
sr-11778.swift:7:5: note: 'x' declared here
let x = DoubleANumber()
    ^

[hamishknight]

@dan-zheng If you don't mind, I'd like to take this. Hopefully the fix should be as simple as updating getCalleeLocator to deal with this case, which I believe should also fix https://bugs.swift.org/browse/SR-11386.

[dan-zheng]

@hamishknight: please go ahead!

[xedin]

Sorry, this completely slipped from my radar :/ I think Hamish is right and it should be fixable by updating `getCalleeLocator`

[hamishknight]

#28519 fixes this for the common case of a regular call, such as the example given in the report. It doesn't however fix this for ".member" exprs, I've filed SR-11909 for that, and plan on fixing in a follow-up.