Issue Description:

This fails to compile:

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?.

However, it compiles if the trailing closure syntax is used:

dict[0] = array.reduce(0) { $0 + $1 } // (B)

or if the initial value is given as Int(0) instead of 0:

dict[0] = array.reduce(Int(0), { $0 + $1 }) // (C)

I do not know if (A) should compile, but there should not be a difference between (A), (B), and (C).