SR-11283: Amend OptionSet.remove method

stdlib/public/core/OptionSet.swift

@@ -286,9 +286,13 @@ extension OptionSet where Element == Self {
   @inlinable // generic-performance
   @discardableResult
   public mutating func remove(_ member: Element) -> Element? {
-    let r = isSuperset(of: member) ? Optional(member) : nil
+    let intersectionElements = intersection(member)
+    guard !intersectionElements.isEmpty else {
+      return nil
+    }
+
     self.subtract(member)
-    return r
+    return intersectionElements
   }
 
   /// Inserts the given element into the set.

test/stdlib/OptionSetTest.swift

@@ -83,7 +83,40 @@ tests.test("basics") {
   expectEqual([.Satchel, .Box], p)
 }
 
-// FIXME: add tests for all of SetAlgebra, in particular
-// insert/remove/replace.
+tests.test("set algebra") {
+  typealias P = PackagingOptions
+
+  // remove
+  var p = P.BoxOrBag
+  expectNil(p.remove(P.Carton))
+
+  p = P.BoxOrBag
+  p.remove(P.BoxOrCartonOrBag)
+  expectEqual(P(), p)
+
+  p = P.BoxOrBag
+  expectEqual(P.Bag, p.remove(P.SatchelOrBag))
+  expectEqual(P.Box, p)
+
+  // insert
+  p = P.Box
+  var insertionResult = p.insert(.Bag)
+  expectTrue(insertionResult.inserted)
+  expectEqual(P.Bag, insertionResult.memberAfterInsert)
+  expectEqual(P.BoxOrBag, p)
+
+  insertionResult = p.insert(.Bag)
+  expectFalse(insertionResult.inserted)
+  expectEqual(P.Bag, insertionResult.memberAfterInsert)
+
+  // update
+  p = P.Box
+  expectNil(p.update(with: .Bag))
+  expectEqual(P.BoxOrBag, p)
+
+  p = P.Box
+  expectEqual(P.Box, p.update(with: .BoxOrBag))
+  expectEqual(P.BoxOrBag, p)
+}
 
 runAllTests()