Merge pull request #4652 from DougGregor/document-error-userinfo-type

Tighten up and document the type of Error._userInfo somewhat.

Author: DougGregor

stdlib/public/SDK/Foundation/NSError.swift

@@ -287,7 +287,7 @@ public func _swift_Foundation_getErrorDefaultUserInfo(_ error: Error)
 extension NSError : Error {
   public var _domain: String { return domain }
   public var _code: Int { return code }
-  public var _userInfo: Any? { return userInfo }
+  public var _userInfo: AnyObject? { return userInfo as NSDictionary }
 
   /// The "embedded" NSError is itself.
   public func _getEmbeddedNSError() -> AnyObject? {
@@ -304,8 +304,8 @@ extension CFError : Error {
     return CFErrorGetCode(self)
   }
 
-  public var _userInfo: Any? {
-    return CFErrorCopyUserInfo(self) as Any
+  public var _userInfo: AnyObject? {
+    return CFErrorCopyUserInfo(self) as AnyObject
   }
 
   /// The "embedded" NSError is itself.

stdlib/public/core/ErrorType.swift

@@ -113,7 +113,11 @@ import SwiftShims
 public protocol Error {
   var _domain: String { get }
   var _code: Int { get }
-  var _userInfo: Any? { get }
+
+  // Note: _userInfo is always an NSDictionary, but we cannot use that type here
+  // because the standard library cannot depend on Foundation. However, the
+  // underscore implies that we control all implementations of this requirement.
+  var _userInfo: AnyObject? { get }
 
 #if _runtime(_ObjC)
   func _getEmbeddedNSError() -> AnyObject?
@@ -203,7 +207,7 @@ extension Error {
     return String(reflecting: type(of: self))
   }
 
-  public var _userInfo: Any? {
+  public var _userInfo: AnyObject? {
 #if _runtime(_ObjC)
     return _stdlib_getErrorDefaultUserInfo(self)
 #else