[Type checker] Warn about overrides of NSObject.hashValue.

NSObject.hashValue is provided to satisfy the hashValue constraint of
the Hashable protocol. However, it is not the correct customization
point for interoperating with Objective-C, because Objective-C code
will call through the -hash method. Warn about overrides of
NSObject.hashValue; users should override NSObject.hash instead.

Fixes rdar://problem/42780635.

(cherry picked from commit cc4c992)

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -4022,6 +4022,10 @@ NOTE(redundant_particular_literal_case_here,none,
 
 WARNING(non_exhaustive_switch_warn,none, "switch must be exhaustive", ())
 
+WARNING(override_nsobject_hashvalue,none,
+        "override of 'NSObject.hashValue' is deprecated; "
+        "override 'NSObject.hash' to get consistent hashing behavior", ())
+
 #ifndef DIAG_NO_UNDEF
 # if defined(DIAG)
 #  undef DIAG

lib/Sema/TypeCheckDecl.cpp

@@ -6119,6 +6119,18 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       diagnoseOverrideForAvailability(TC, override, base);
     }
 
+    // Overrides of NSObject.hashValue are deprecated; one should override
+    // NSObject.hash instead.
+    if (auto baseVar = dyn_cast<VarDecl>(base)) {
+      if (auto classDecl =
+          baseVar->getDeclContext()->getAsClassOrClassExtensionContext()) {
+        if (classDecl->getBaseName().userFacingName() == "NSObject" &&
+            baseVar->getBaseName().userFacingName() == "hashValue") {
+          override->diagnose(diag::override_nsobject_hashvalue);
+        }
+      }
+    }
+
     /// Check attributes associated with the base; some may need to merged with
     /// or checked against attributes in the overriding declaration.
     AttributeOverrideChecker attrChecker(TC, base, override);

test/ClangImporter/objc_override.swift

@@ -108,6 +108,14 @@ class CallbackSubC : CallbackBase {
   override func perform(optNonescapingHandler: @escaping () -> Void) {} // expected-error {{method does not override any method from its superclass}}
 }
 
+//
+class MyHashableNSObject: NSObject {
+  override var hashValue: Int { // expected-warning{{override of 'NSObject.hashValue' is deprecated}}
+    return 0
+  }
+}
+
+
 // FIXME: Remove -verify-ignore-unknown.
 // <unknown>:0: error: unexpected note produced: overridden declaration is here
 // <unknown>:0: error: unexpected note produced: setter for 'boolProperty' declared here

test/Inputs/clang-importer-sdk/swift-modules/Foundation.swift

@@ -2,8 +2,14 @@
 @_exported import CoreGraphics
 @_exported import Foundation
 
-public func == (lhs: NSObject, rhs: NSObject) -> Bool {
-  return lhs.isEqual(rhs)
+extension NSObject : Equatable, Hashable {
+  @objc open var hashValue: Int {
+    return hash
+  }
+
+  public static func == (lhs: NSObject, rhs: NSObject) -> Bool {
+    return lhs.isEqual(rhs)
+  }
 }
 
 public let NSUTF8StringEncoding: UInt = 8

test/Inputs/clang-importer-sdk/swift-modules/ObjectiveC.swift

@@ -86,13 +86,3 @@ public func _convertObjCBoolToBool(_ x: ObjCBool) -> Bool {
 public func ~=(x: NSObject, y: NSObject) -> Bool {
   return true
 }
-
-extension NSObject : Equatable, Hashable {
-  public var hashValue: Int {
-    return hash
-  }
-}
-
-public func == (lhs: NSObject, rhs: NSObject) -> Bool {
-  return lhs.isEqual(rhs)
-}