fix: adds topology discovery for sharded cluster

NODE-2827

lib/core/uri_parser.js

@@ -687,15 +687,6 @@ 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/sharding_connection.test.js

@@ -1,13 +1,29 @@
 'use strict';
 
+const withClient = require('./shared').withClient;
 const setupDatabase = require('./shared').setupDatabase;
 const expect = require('chai').expect;
+const TopologyType = require('../../lib/core/sdam/common').TopologyType;
 
 describe('Sharding (Connection)', function() {
   before(function() {
     return setupDatabase(this.configuration);
   });
 
+  it('Should use sharded topology', {
+    metadata: { requires: { topology: 'sharded' } },
+    test: function() {
+      const client = this.configuration.newClient({}, { useUnifiedTopology: true });
+      return withClient(client, (client, done) => {
+        expect(client).to.exist;
+        expect(client)
+          .nested.property('topology.description.type')
+          .to.equal(TopologyType.Sharded);
+        return done();
+      })();
+    }
+  });
+
   /**
    * @ignore
    */