[SE-0112] Make NSCustomError.errorDomain static.

The error domain is meant to identify the type; it cannot meaningfully vary.

Author: DougGregor

stdlib/public/SDK/Foundation/NSError.swift

@@ -118,7 +118,7 @@ public extension RecoverableError {
 /// and user-info dictionary.
 public protocol CustomNSError : ErrorProtocol {
   /// The domain of the error.
-  var errorDomain: String { get }
+  static var errorDomain: String { get }
 
   /// The error code within the given domain.
   var errorCode: Int { get }
@@ -129,7 +129,7 @@ public protocol CustomNSError : ErrorProtocol {
 
 public extension ErrorProtocol where Self : CustomNSError {
   /// Default implementation for customized NSErrors.
-  var _domain: String { return self.errorDomain }
+  var _domain: String { return Self.errorDomain }
 
   /// Default implementation for customized NSErrors.
   var _code: Int { return self.errorCode }
@@ -378,7 +378,7 @@ public extension _BridgedStoredNSError {
   // FIXME: Would prefer to have a clear "extract an NSError
   // directly" operation.
 
-  var errorDomain: String { return _nsError.domain }
+  static var errorDomain: String { return _nsErrorDomain }
 
   var errorCode: Int { return _nsError.code }
 
@@ -428,6 +428,7 @@ public struct NSCocoaError : _BridgedStoredNSError {
   public let _nsError: NSError
 
   public init(_nsError error: NSError) {
+    precondition(error.domain == NSCocoaErrorDomain)
     self._nsError = error
   }
 
@@ -1168,6 +1169,7 @@ public struct NSURLError : _BridgedStoredNSError {
   public let _nsError: NSError
 
   public init(_nsError error: NSError) {
+    precondition(error.domain == NSURLErrorDomain)
     self._nsError = error
   }
 

test/1_stdlib/ErrorProtocolBridging.swift

@@ -335,7 +335,6 @@ ErrorProtocolBridgingTests.test("Thrown NSError identity is preserved") {
 
 // Check errors customized via protocol.
 struct MyCustomizedError : ErrorProtocol {
-  let domain: String
   let code: Int
 }
 
@@ -358,8 +357,8 @@ extension MyCustomizedError : LocalizedError {
 }
 
 extension MyCustomizedError : CustomNSError {
-  var errorDomain: String {
-    return domain
+  static var errorDomain: String {
+    return "custom"
   }
 
   /// The error code within the given domain.
@@ -418,7 +417,7 @@ class RecoveryDelegate {
 }
 
 ErrorProtocolBridgingTests.test("Customizing NSError via protocols") {
-  let error = MyCustomizedError(domain: "custom", code: 12345)
+  let error = MyCustomizedError(code: 12345)
   let nsError = error as NSError
 
   // CustomNSError