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

[Martin R]

> 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 .