Apply KeyPath in map() in simplest form

let test = [4, 5, 3, 1, 3]
print(
    test.map { $0 }
)
print(
    test.map(\.self)  // Doesn't compile  
)

Error:

Type of expression is ambiguous without more context

Why doesn't it work? Seems like it should.
If isn't this way, how else can we rid of the ugly { $0 } here?

Maybe an example with compactMap will make more cense))

let test = [4, 5, nil, 3, 1, nil, 3]
print(
    test.compactMap { $0 }
)
print(
    test.compactMap(\.self)  // Doesn't compile  
)

Error:

Cannot convert value of type 'WritableKeyPath<_, _>' to expected argument type '(Int?) throws -> ElementOfResult?'

Why doesn't it work? Seems like it should.

You are right. Both of your examples should compile because

• SE-0227 Identity Keypath has been implemented in Swift 5, and
• SE-0249 Key Path Expressions as Functions has been implemented in Swift 5.2.

In fact the latter proposal explicitly mentions an example similar to yours. However, that does not compile (as of Xcode 11.7 and Xcode 12 beta):

[1, nil, 3, nil, 5].compactMap(\.self)
// error: generic parameter 'ElementOfResult' could not be inferred

That is a bug. It has already been reported as

• SR-12897 - ".self" not working with keypath-as-function - Examples in Key Path Expressions as Functions proposal don't compile.

The bug report mentions a custom extension as a possible workaround:

extension Optional { var mySelf: Self { self } }

[1, nil, 3, nil, 5].compactMap(\.mySelf)