[TypeChecker] NFC: Add a test-case for rdar://109381194

Author: xedin

test/Constraints/casts.swift

@@ -718,3 +718,26 @@ func isHashable_is(_ error: Error) -> Bool {
 func isHashable_composition(_ error: Error & AnyObject) -> Bool {
   error is AnyHashable // OK
 }
+
+// rdar://109381194 - incorrect ambiguity while comparing collections
+do {
+  class A : Hashable, Equatable {
+    func hash(into hasher: inout Hasher) {
+    }
+
+    static func == (lhs: A, rhs: A) -> Bool {
+      false
+    }
+  }
+
+  class B : A {}
+
+  func test(a: [B], b: [String: B]) {
+    assert(Set(a) == Set(b.values.map { $0 as A })) // ok
+  }
+
+  func test(a: [A], b: [B]) {
+    assert(Set(a) == Set(b.map { $0 as A })) // Ok
+    assert(Set(b.map { $0 as A }) == Set(a)) // Ok
+  }
+}