Understanding the limitations of open existential

Using Swift
3

Thanks. You gave me a clue. :smiley: But I still am confused! :sweat_smile:

let collections: [any Collection<Int>] = [[1,2,3], [1,2,3]]
print(type(of: collections.flatMap { $0 }))

This prints:

swift test2.swift
test2.swift:2:28: warning: 'flatMap' is deprecated: Please use compactMap(_:) for the case where closure returns an optional value
print(type(of: collections.flatMap { $0 }))
                           ^
test2.swift:2:28: note: use 'compactMap(_:)' instead
print(type(of: collections.flatMap { $0 }))
                           ^~~~~~~
                           compactMap
<<< invalid type >>>

If I go ahead and change it to compactMap, the program prints:

<<< invalid type >>>

I was hoping for it to print [Int].

FWIW, Swift version 5.8 (swiftlang-5.8.0.124.2 clang-1403.0.22.11.100). I am using a playground but behavior is the same for the command line.