Add AnyHashable support for Foundation.Decimal

Add a custom AnyHashable representation to Foundation.Decimal, so that
it behaves like all other bridged types.

Author: gribozavr

stdlib/public/SDK/Foundation/Foundation.swift

@@ -433,6 +433,9 @@ extension NSNumber : _HasCustomAnyHashableRepresentation {
     guard let kind = _SwiftTypePreservingNSNumberTag(
       rawValue: Int(_swift_Foundation_TypePreservingNSNumberGetKind(self))
     ) else {
+      if let nsDecimalNumber: NSDecimalNumber = self as? NSDecimalNumber {
+        return AnyHashable(nsDecimalNumber as Decimal)
+      }
       return nil
     }
     switch kind {

stdlib/public/SDK/Foundation/TypePreservingNSNumber.mm

@@ -30,8 +30,6 @@ @interface _SwiftTypePreservingNSNumber : NSNumber
   SwiftDouble = 4,
   SwiftCGFloat = 5,
   SwiftBool = 6
-  // FIXME(id-as-any): add support for NSDecimal and
-  // Foundation.Decimal.
 };
 
 @implementation _SwiftTypePreservingNSNumber {

validation-test/stdlib/NSDecimalNumberBridging.swift.gyb

@@ -0,0 +1,25 @@
+// RUN: %target-run-simple-swiftgyb
+// REQUIRES: executable_test
+// REQUIRES: objc_interop
+
+import Foundation
+import StdlibUnittest
+
+let NSDecimalNumberTests = TestSuite("NSDecimalNumber")
+
+NSDecimalNumberTests.test("AnyHashable containing Foundation.Decimal") {
+  let values = [
+    NSDecimalNumber(string: "10.0"),
+    NSDecimalNumber(string: "20.0"),
+    NSDecimalNumber(string: "20.0"),
+  ]
+  let anyHashables = values.map(AnyHashable.init)
+  expectEqual("Decimal", String(anyHashables[0].base.dynamicType))
+  expectEqual("Decimal", String(anyHashables[1].base.dynamicType))
+  expectEqual("Decimal", String(anyHashables[2].base.dynamicType))
+  expectNotEqual(anyHashables[0], anyHashables[1])
+  expectEqual(anyHashables[1], anyHashables[2])
+}
+
+runAllTests()
+