Proposal: Conform Never to Codable

proposals/0396-never-codable.md

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+# Conform `Never` to `Codable`
+
+* Proposal: [SE-0396](0396-never-codable.md)
+* Author: [Nate Cook](https://github.com/natecook1000)
+* Review Manager: [Tony Allevato](https://github.com/allevato)
+* Status: **Active Review (April 18...May 3, 2023)**
+* Implementation: [apple/swift#64899](https://github.com/apple/swift/pull/64899)
+* Review: ([pitch](https://forums.swift.org/t/pitch-conform-never-to-codable/64056))
+
+## Introduction
+
+Extend `Never` so that it conforms to the `Encodable` and `Decodable` protocols, together known as `Codable`.
+
+## Motivation
+
+Swift can synthesize `Codable` conformance for any type that has `Codable` members. Generic types often participate in this synthesized conformance by constraining their generic parameters, like this `Either` type:
+
+```swift
+enum Either<A, B> {
+    case left(A)
+    case right(B)
+}
+
+extension Either: Codable where A: Codable, B: Codable {}
+```
+
+In this way, `Either` instances where both generic parameters are `Codable` are `Codable` themselves, such as an `Either<Int, Double>`. However, since `Never` isn't `Codable`, using `Never` as one of the parameters blocks the conditional conformance, even though it would be perfectly fine to encode or decode a type like `Either<Int, Never>`.
+
+## Proposed solution
+
+The standard library should add `Encodable` and `Decodable` conformance to the `Never` type.
+
+## Detailed design
+
+The `Encodable` conformance is simple — since it's impossible to have a `Never` instance, the `encode(to:)` method can simply be empty.
+
+The `Decodable` protocol requires the `init(from:)` initializer, which clearly can't create a `Never` instance. Because trying to decode invalid input isn't a programmer error, a fatal error would be inappropriate. Instead, the implementation throws a `DecodingError.dataCorrupted` error if decoding is attempted.
+
+## Source compatibility
+
+If existing code already declares `Codable` conformance, that code will begin to emit a warning: e.g. `Conformance of 'Never' to protocol 'Encodable' was already stated in the type's module 'Swift'`.
+
+The new conformance shouldn't differ from existing conformances, since it isn't possible to construct an instance of `Never`.
+
+## ABI compatibility
+
+The proposed change is additive and does not change any existing ABI.
+
+## Implications on adoption
+
+The new conformance will have availability annotations.
+
+## Future directions
+
+None.
+
+## Alternatives considered
+
+None.