This post is a little bit the information aggregator. If you find a mistake, you could write to me about it I really appreciate that. Have a nice read.

Example with JSONDecoder

What would happen if we run the following piece of code?

struct Test<T>: Codable where T: Codable {
    enum CodingKeys: String, CodingKey {
        case value
    }
    
    let value: T
    let info: String
}

extension Test {
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.value = try container.decode(T.self, forKey: .value)
        self.info = "Default init(from decoder:)"
    }
}

extension Test where T == String {
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.value = try container.decode(T.self, forKey: .value)
        self.info = "Custom init(from decoder:)"
    }
}

let data = #"{"value":"Hello, World!"}"#.data(using: .utf8)!
let object = try? JSONDecoder().decode(Test<String>.self, from: data)
print(object.debugDescription)

Try thinking for 5 seconds about the result.

The result

Optional(
    Test<String>(
        value: "Hello, World!", 
        info: "Default init(from decoder:)"
    )
)

Why did it happen?

The JSONDecoder:decode definition looks like

func decode<T>(_ type: T.Type, from data: Data) throws -> T where T: Decodable

We see the generic function and also the metatype T.Type. I’m not focusing your attention on those two definitions by accident. We should understand these structures of language.

You could read more about metatypes:

Consider the example with metatype.

protocol TestProtocol {
    var info: String { get }
    init(from value: Codable)
}

struct Test<T: Codable>: TestProtocol {
    let value: T
    let info: String
}

extension Test {
    init(from value: Codable) {
        self.value = value as! T
        self.info = "Default init(value:)"
    }
}

extension Test where T == String {
    init(from value: Codable) {
        self.value = value as! T
        self.info = "Custom init(value:)"
    }
}

let type: TestProtocol.Type = Test<String>.self
print(type.init(from: "Hello, World!").info)

We’ll get the "Default init(value:)". The reason is the second init(from value: Codable) not requirements of such protocol because for the swift compiler it’s just another method. However, it’s overloading of the method for us.

These methods calls static (it isn’t about static func). Generally, the Static dispatch works here — the swift compiler discribes how a programm will select which implementation of a method on the compile time.

You will see that if you build a Swift Intermediate Language (SIL) file by the example.

> swiftc -emit-sil example.swift > example.swift.sil

No polymorphism for static methods.

Where a same problem could be in JSONDecoder:decode? If we see how it works, we will find the reason. The next code from the official repository.

open func decode<T : Decodable>(_ type: T.Type, from data: Data) throws -> T {
    let topLevel: Any
    do {
        topLevel = try JSONSerialization.jsonObject(with: data)
    } catch {
        throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: [], debugDescription: "The given data was not valid JSON.", underlyingError: error))
    }

    let decoder = __JSONDecoder(referencing: topLevel, options: self.options)
    guard let value = try decoder.unbox(topLevel, as: type) else {
        throw DecodingError.valueNotFound(type, DecodingError.Context(codingPath: [], debugDescription: "The given data did not contain a top-level value."))
    }

    return value
}

// MARK: - Concrete Value Representations
private extension __JSONDecoder {
    
    ...

    func unbox<T : Decodable>(_ value: Any, as type: T.Type) throws -> T? {
        return try unbox_(value, as: type) as? T
    }

    func unbox_(_ value: Any, as type: Decodable.Type) throws -> Any? {
        ... {
            return try type.init(from: self)
        }
    }
}

You would think a problem will be when the unbox_ called, but the situation a little bit complicated.
Consider another example:

func generate<T: TestProtocol, Value: Codable>(value: Value, as type: T.Type) -> T {
    type.init(from: value)
}

print(generate(value: "Hello, World!", as: Test<String>.self).info)

We’ll get the "Default init(value:)" again. What will we see in the SIL code for the generate function?

// generate<A, B>(value:as:)
sil hidden @$s5test28generate5value2asxq__xmtAA12TestProtocolRzSeR_SER_r0_lF : $@convention(thin) <T, Value where T : TestProtocol, Value : Decodable, Value : Encodable> (@in_guaranteed Value, @thick T.Type) -> @out T {
// %0                                             // user: %9
// %1                                             // users: %7, %3
// %2                                             // users: %9, %4
bb0(%0 : $*T, %1 : $*Value, %2 : $@thick T.Type):
  debug_value_addr %1 : $*Value, let, name "value", argno 1 // id: %3
  debug_value %2 : $@thick T.Type, let, name "type", argno 2 // id: %4
  %5 = alloc_stack $Decodable & Encodable         // users: %10, %9, %6
  %6 = init_existential_addr %5 : $*Decodable & Encodable, $Value // user: %7
  copy_addr %1 to [initialization] %6 : $*Value   // id: %7
  %8 = witness_method $T, #TestProtocol.init!allocator.1 : <Self where Self : TestProtocol> (Self.Type) -> (Decodable & Encodable) -> Self : $@convention(witness_method: TestProtocol) <τ_0_0 where τ_0_0 : TestProtocol> (@in Decodable & Encodable, @thick τ_0_0.Type) -> @out τ_0_0 // user: %9
  %9 = apply %8<T>(%0, %5, %2) : $@convention(witness_method: TestProtocol) <τ_0_0 where τ_0_0 : TestProtocol> (@in Decodable & Encodable, @thick τ_0_0.Type) -> @out τ_0_0
  dealloc_stack %5 : $*Decodable & Encodable      // id: %10
  %11 = tuple ()                                  // user: %12
  return %11 : $()                                // id: %12
} // end sil function '$s5test28generate5value2asxq__xmtAA12TestProtocolRzSeR_SER_r0_lF'

As we see the generate function works with TestProtocol.init. Why? You could read the small article about the Abstract Difference of SIL Types. I just show you three base things about generics’ working as I’ve understood this:

Don’t generate a different copy of generic function for every unconstrained type.
Don’t give every type in the language a common representation.
Don’t dynamically construct a call to generator depending on an unconstrained type.

I hope this information will help you.

References

What’s .self, .Type and .Protocol?
WWDC 2015: Protocol-Oriented Programming in Swift
WWDC 2016: Understanding Swift Performance
Swift Protocol Extensions Method Dispatch
Static vs Dynamic Dispatch in Swift: A decisive choice
Swift Intermediate Language (SIL)

Confusing extensions in Swift