Added test that Optional.none is correctly cast in a generic context.

Author: mattpolzin

test/stdlib/Casts.swift

@@ -43,7 +43,7 @@ CastsTests.test("No overrelease of existential boxes in failed casts") {
             }
         }
     }
-    
+
     let err: Error = ErrClass()
     bar(err)
 }
@@ -70,6 +70,16 @@ CastsTests.test("Multi-level optionals can be casted") {
   testFailure(42, from: Int???.self, to: String.self)
 }
 
+// Test for SR-9837: Optional<T>.none not casting to Optional<U>.none in generic context
+CastsTests.test("Optional<T>.none can be casted to Optional<U>.none in generic context") {
+  func test<T>(_ type: T.Type) -> T? {
+    return Any?.none as? T
+  }
+
+  expectEqual(type(of: test(Bool.self)), Bool?.self)
+  expectEqual(type(of: test(Bool?.self)), Bool??.self)
+}
+
 #if _runtime(_ObjC)
 extension CFBitVector : P {
   static func makeImmutable(from values: Array<UInt8>) -> CFBitVector {