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

[Joe Groff]

> 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