Swift compactMap includes nil value

Consider the following:

let numbers: [String?] = ["1", "two", nil]
let intArray: [Int?] = numbers.compactMap {Int($0 ?? "0")}
print(intArray)

The result printed is [Optional(1), nil, Optional(0)], why does it include the nil value? I thought these would be filtered out because of compactMap.

Interestingly let intArray: [Int] = numbers.compactMap {Int($0 ?? "0")} works without an issue.

This is a good example why explicit type annotation makes things sometimes worse.

According to the definition compactMap returns a non-optional array.

Without type annotation

let intArray = numbers.compactMap {Int($0 ?? "0")}

or with the correct type annotation

let intArray : [Int] = numbers.compactMap {Int($0 ?? "0")}

the result is

[1, 0]

• "1" is mapped to 1
• "two" is ignored
• nil is nil-coalesced to "0" and mapped to 0

With the bad/wrong type annotation [Int?] the following happens

• "1" is mapped to Optional(1)
• "two" cannot be converted to Int so instead of ignoring the value nil is returned
• nil is nil-coalesced to "0" and mapped to Optional(0)