[swift-users] Type inference when assigning the result of reduce to a dictionary

[Joe Groff]

> On Oct 4, 2016, at 1:58 PM, Martin R <martinr448 at gmail.com> wrote:
> 
> 
>> On 4 Oct 2016, at 21:42, Joe Groff <jgroff at apple.com> wrote:
>> 
>> 
>>> On Oct 4, 2016, at 5:20 AM, Martin R via swift-users <swift-users at swift.org> wrote:
>>> 
>>> I noticed the following when assigning the result of `reduce()` to a dictionary:
>>> 
>>>  let array = [1, 2, 3]
>>>  var dict: [Int: Int] = [:]
>>>  dict[0] = array.reduce(0, { $0 + $1 }) // (A)
>>>  // error: binary operator '+' cannot be applied to operands of
>>> type 'Int?' and 'Int'
>>>  // dict[0] = array.reduce(0, { $0 + $1 })
>>>  //                             ~~ ^ ~~
>>> 
>>> It seems that the compiler tries to make the RHS an `Int?` and
>>> therefore infers the type of the initial value `0` and the
>>> accumulating value `$0` as `Int?`.
>>> 
>>> That is in some sense correct, since the dictionary subscript setter
>>> takes an optional as parameter, in this case `Int?`.
>>> 
>>> However, the code compiles (and runs as expected) if the trailing
>>> closure syntax is used:
>>> 
>>>  dict[0] = array.reduce(0) { $0 + $1 } // (B)
>>> 
>>> and also if the initial value is given as `0` instead of `Int(0)`:
>>> 
>>>  dict[0] = array.reduce(Int(0), { $0 + $1 }) // (C)
>>> 
>>> My questions are:
>>> - Should (A) compile?
>>> - Why does it make a difference if the trailing closure syntax is used
>>> (A vs. B)?
>>> - Why does it make a difference if the initial value is given as `0`
>>> or `Int(0)` (A vs. C)?
>> 
>> No good reason. Got time to file a bug?
>> 
>> -Joe
> 
> Done: https://bugs.swift.org/browse/SR-2853 .

Thanks!

-Joe