fix: restore auto direct connection behavior

lib/core/uri_parser.js

@@ -687,6 +687,15 @@ function parseConnectionString(uri, options, callback) {
     return callback(new MongoParseError('directConnection option requires exactly one host'));
   }
 
+  // NOTE: this behavior will go away in v4.0, we will always auto discover there
+  if (
+    parsedOptions.directConnection == null &&
+    hosts.length === 1 &&
+    parsedOptions.replicaSet == null
+  ) {
+    parsedOptions.directConnection = true;
+  }
+
   const result = {
     hosts: hosts,
     auth: auth.db || auth.username ? auth : null,

test/functional/mongo_client_options.test.js

@@ -168,6 +168,20 @@ describe('MongoClient Options', function() {
     }
   });
 
+  it('should directConnect when no replicaSet name is specified', {
+    metadata: { requires: { topology: 'replicaset' } },
+    test: function(done) {
+      // strip the replicaSet parameter from the url if present
+      const urlNoReplicaSet = this.configuration.url().replace(/([&?])replicaSet=.+?[&$]/, '$1');
+      const client = this.configuration.newClient(urlNoReplicaSet);
+      client.connect(err => {
+        expect(err).to.not.exist;
+        expect(client.s.options.directConnection).to.be.true;
+        client.close(done);
+      });
+    }
+  });
+
   /**
    * @ignore
    */

test/functional/sharding_connection.test.js

@@ -13,7 +13,12 @@ describe('Sharding (Connection)', function() {
   it('Should use sharded topology', {
     metadata: { requires: { topology: 'sharded' } },
     test: function() {
-      const client = this.configuration.newClient({}, { useUnifiedTopology: true });
+      const client = this.configuration.newClient(
+        {},
+        // note: auto-discovery will be the default behavior in 4.0,
+        // for now we must supply directConnection: false
+        { useUnifiedTopology: true, directConnection: false }
+      );
       return withClient(client, (client, done) => {
         expect(client).to.exist;
         expect(client)