Merge pull request swiftlang#10470 from itaiferber/codable-improved-runtime-diagnostics

Improve runtime errors for conditionally-Codable types

Author: itaiferber

stdlib/public/core/Codable.swift

@@ -3233,15 +3233,33 @@ public extension RawRepresentable where RawValue == String, Self : Decodable {
 }
 
 //===----------------------------------------------------------------------===//
-// Optional Conformance
+// Optional/Collection Type Conformances
 //===----------------------------------------------------------------------===//
 
+fileprivate func assertTypeIsEncodable<T>(_ type: T.Type, in wrappingType: Any.Type) {
+    guard T.self is Encodable.Type else {
+        if T.self == Encodable.self || T.self == Codable.self {
+            preconditionFailure("\(wrappingType) does not conform to Encodable because Encodable does not conform to itself. You must use a concrete type to encode or decode.")
+        } else {
+            preconditionFailure("\(wrappingType) does not conform to Encodable because \(T.self) does not conform to Encodable.")
+        }
+    }
+}
+
+fileprivate func assertTypeIsDecodable<T>(_ type: T.Type, in wrappingType: Any.Type) {
+    guard T.self is Decodable.Type else {
+        if T.self == Decodable.self || T.self == Codable.self {
+            preconditionFailure("\(wrappingType) does not conform to Decodable because Decodable does not conform to itself. You must use a concrete type to encode or decode.")
+        } else {
+            preconditionFailure("\(wrappingType) does not conform to Decodable because \(T.self) does not conform to Decodable.")
+        }
+    }
+}
+
 // FIXME: Uncomment when conditional conformance is available.
 extension Optional : Encodable /* where Wrapped : Encodable */ {
     public func encode(to encoder: Encoder) throws {
-        guard Wrapped.self is Encodable.Type else {
-            preconditionFailure("\(type(of: self)) does not conform to Encodable because \(Wrapped.self) does not conform to Encodable.")
-        }
+        assertTypeIsEncodable(Wrapped.self, in: type(of: self))
 
         var container = encoder.singleValueContainer()
         switch self {
@@ -3253,12 +3271,9 @@ extension Optional : Encodable /* where Wrapped : Encodable */ {
 
 extension Optional : Decodable /* where Wrapped : Decodable */ {
     public init(from decoder: Decoder) throws {
-        // Initialize self here so we can print type(of: self).
+        // Initialize self here so we can get type(of: self).
         self = .none
-
-        guard Wrapped.self is Decodable.Type else {
-            preconditionFailure("\(type(of: self)) does not conform to Decodable because \(Wrapped.self) does not conform to Decodable.")
-        }
+        assertTypeIsDecodable(Wrapped.self, in: type(of: self))
 
         let container = try decoder.singleValueContainer()
         if !container.decodeNil() {
@@ -3269,16 +3284,10 @@ extension Optional : Decodable /* where Wrapped : Decodable */ {
     }
 }
 
-//===----------------------------------------------------------------------===//
-// Collection Conformances
-//===----------------------------------------------------------------------===//
-
 // FIXME: Uncomment when conditional conformance is available.
 extension Array : Encodable /* where Element : Encodable */ {
     public func encode(to encoder: Encoder) throws {
-        guard Element.self is Encodable.Type else {
-            preconditionFailure("\(type(of: self)) does not conform to Encodable because \(Element.self) does not conform to Encodable.")
-        }
+        assertTypeIsEncodable(Element.self, in: type(of: self))
 
         var container = encoder.unkeyedContainer()
         for element in self {
@@ -3292,12 +3301,9 @@ extension Array : Encodable /* where Element : Encodable */ {
 
 extension Array : Decodable /* where Element : Decodable */ {
     public init(from decoder: Decoder) throws {
-        // Initialize self here so we can print type(of: self).
+        // Initialize self here so we can get type(of: self).
         self.init()
-
-        guard Element.self is Decodable.Type else {
-            preconditionFailure("\(type(of: self)) does not conform to Decodable because \(Element.self) does not conform to Decodable.")
-        }
+        assertTypeIsDecodable(Element.self, in: type(of: self))
 
         let metaType = (Element.self as! Decodable.Type)
         var container = try decoder.unkeyedContainer()
@@ -3313,9 +3319,7 @@ extension Array : Decodable /* where Element : Decodable */ {
 
 extension Set : Encodable /* where Element : Encodable */ {
     public func encode(to encoder: Encoder) throws {
-        guard Element.self is Encodable.Type else {
-            preconditionFailure("\(type(of: self)) does not conform to Encodable because \(Element.self) does not conform to Encodable.")
-        }
+        assertTypeIsEncodable(Element.self, in: type(of: self))
 
         var container = encoder.unkeyedContainer()
         for element in self {
@@ -3329,12 +3333,9 @@ extension Set : Encodable /* where Element : Encodable */ {
 
 extension Set : Decodable /* where Element : Decodable */ {
     public init(from decoder: Decoder) throws {
-        // Initialize self here so we can print type(of: self).
+        // Initialize self here so we can get type(of: self).
         self.init()
-
-        guard Element.self is Decodable.Type else {
-            preconditionFailure("\(type(of: self)) does not conform to Decodable because \(Element.self) does not conform to Decodable.")
-        }
+        assertTypeIsDecodable(Element.self, in: type(of: self))
 
         let metaType = (Element.self as! Decodable.Type)
         var container = try decoder.unkeyedContainer()
@@ -3366,13 +3367,8 @@ internal struct _DictionaryCodingKey : CodingKey {
 
 extension Dictionary : Encodable /* where Key : Encodable, Value : Encodable */ {
     public func encode(to encoder: Encoder) throws {
-        guard Key.self is Encodable.Type else {
-            preconditionFailure("\(type(of: self)) does not conform to Encodable because \(Key.self) does not conform to Encodable.")
-        }
-
-        guard Value.self is Encodable.Type else {
-            preconditionFailure("\(type(of: self)) does not conform to Encodable because \(Value.self) does not conform to Encodable.")
-        }
+        assertTypeIsEncodable(Key.self, in: type(of: self))
+        assertTypeIsEncodable(Value.self, in: type(of: self))
 
         if Key.self == String.self {
             // Since the keys are already Strings, we can use them as keys directly.
@@ -3411,14 +3407,8 @@ extension Dictionary : Decodable /* where Key : Decodable, Value : Decodable */
     public init(from decoder: Decoder) throws {
         // Initialize self here so we can print type(of: self).
         self.init()
-
-        guard Key.self is Decodable.Type else {
-            preconditionFailure("\(type(of: self)) does not conform to Decodable because \(Key.self) does not conform to Decodable.")
-        }
-
-        guard Value.self is Decodable.Type else {
-            preconditionFailure("\(type(of: self)) does not conform to Decodable because \(Value.self) does not conform to Decodable.")
-        }
+        assertTypeIsDecodable(Key.self, in: type(of: self))
+        assertTypeIsDecodable(Value.self, in: type(of: self))
 
         if Key.self == String.self {
             // The keys are Strings, so we should be able to expect a keyed container.