fix enumeration + tests

Author: gusty

src/FSharpPlus/Extensions/Dict.fs

@@ -187,8 +187,8 @@ module Dict =
                 member __.Count = System.Int32.MaxValue
                 member __.ContainsKey (_key: 'TKey) = true
                 member __.Contains (item: KeyValuePair<'TKey,'TValue>) = obj.ReferenceEquals (item.Value, source)
-                member __.GetEnumerator () = Seq.empty.GetEnumerator () :> System.Collections.IEnumerator
-                member __.GetEnumerator () = Seq.empty.GetEnumerator () : IEnumerator<KeyValuePair<'TKey,'TValue>>
+                member __.GetEnumerator () = invalidOp "Key set is potentially infinite." : System.Collections.IEnumerator
+                member __.GetEnumerator () = invalidOp "Key set is potentially infinite." : IEnumerator<KeyValuePair<'TKey,'TValue>>
                 member __.IsReadOnly = true
                 member __.Values = icollection source
                 member __.Item

tests/FSharpPlus.Tests/General.fs

@@ -436,6 +436,18 @@ module Functor =
         Assert.IsInstanceOf<Option<Async<int>>> (Some testVal10)
         areEqual 2 (testVal10 |> Async.RunSynchronously)
 
+        let testVal11 = (+) "h" <!> dict [1, "i"; 2, "ello"]
+        CollectionAssert.AreEqual (dict [(1, "hi"); (2, "hello")], testVal11)
+
+        let testVal12 =
+            let h: IDictionary<int, string> = result "h"
+            try
+                (+) <!> h <*> dict [1, "i"; 2, "ello"]
+            with _ -> dict [0, "failure"]
+        CollectionAssert.AreEqual (dict [0, "failure"], testVal12)
+
+
+
     [<Test>]
     let unzip () = 
         let testVal = unzip {Head = (1, 'a'); Tail = [(2, 'b');(3, 'b')]}