Decimal: Modernize hashing

Author: lorentey

Foundation/Decimal.swift

@@ -286,8 +286,12 @@ extension Decimal : Hashable, Comparable {
         return Int64(bitPattern: uint64Value)
     }
 
-    public var hashValue: Int {
-        return Int(bitPattern: __CFHashDouble(doubleValue))
+    public func hash(into hasher: inout Hasher) {
+        // FIXME: This is a weak hash.  We should rather normalize self to a
+        // canonical member of the exact same equivalence relation that
+        // NSDecimalCompare implements, then simply feed all components to the
+        // hasher.
+        hasher.combine(doubleValue)
     }
 
     public static func ==(lhs: Decimal, rhs: Decimal) -> Bool {

TestFoundation/TestDecimal.swift

@@ -363,9 +363,10 @@ class TestDecimal: XCTestCase {
         XCTAssertFalse(Decimal.nan.isTotallyOrdered(belowOrEqualTo: Decimal(2.3)))
         XCTAssertTrue(Decimal(2) < Decimal(3))
         XCTAssertTrue(Decimal(3) > Decimal(2))
-#if !arch(arm)
-        XCTAssertEqual(3275573729074, Decimal(1234).hashValue)
-#endif
+
+        // FIXME: This test is of questionable value. We should test hash properties.
+        XCTAssertEqual((1234 as Double).hashValue, Decimal(1234).hashValue)
+
         XCTAssertEqual(Decimal(-9), Decimal(1) - Decimal(10))
         XCTAssertEqual(Decimal(3), Decimal(2).nextUp)
         XCTAssertEqual(Decimal(2), Decimal(3).nextDown)