Fix deep equal comparison between promises

.eslintrc.yaml

@@ -12,6 +12,7 @@ globals:
   Set: false
   Symbol: false
   BigInt: false
+  Promise: false
   Int8Array: false
   Uint8Array: false
   Uint8ClampedArray: false

lib/deep-equal.js

@@ -117,6 +117,10 @@ function deepEqualCyclic(actual, expectation, match) {
             }
         }
 
+        if (actualObj instanceof Promise && expectationObj instanceof Promise) {
+            return actualObj === expectationObj;
+        }
+
         if (actualObj instanceof Error && expectationObj instanceof Error) {
             return actualObj === expectationObj;
         }

lib/deep-equal.test.js

@@ -903,4 +903,19 @@ describe("deepEqual", function () {
             });
         });
     });
+
+    describe("promises", function () {
+        it("returns true if the same promise instance is compared", function () {
+            var promise = Promise.resolve();
+            var checkDeep = samsam.deepEqual(promise, promise);
+            assert.isTrue(checkDeep);
+        });
+
+        it("returns false if a different promise instance is compared", function () {
+            var promise1 = Promise.resolve();
+            var promise2 = Promise.resolve();
+            var checkDeep = samsam.deepEqual(promise1, promise2);
+            assert.isFalse(checkDeep);
+        });
+    });
 });