mongos> db.bar.createIndex( { _id: 1, x: 1, c: 1}, { unique: true });
{
	"raw" : {
		"shardA/Kays-MacBook-Pro-2.local:27018,Kays-MacBook-Pro-2.local:27019,Kays-MacBook-Pro-2.local:27020" : {
			"createdCollectionAutomatically" : false,
			"numIndexesBefore" : 4,
			"numIndexesAfter" : 5,
			"ok" : 1
		},
		"shardB/Kays-MacBook-Pro-2.local:27021,Kays-MacBook-Pro-2.local:27022,Kays-MacBook-Pro-2.local:27023" : {
			"ok" : 0,
			"errmsg" : "request doesn't allow collection to be created implicitly",
			"code" : 227,
			"codeName" : "CannotImplicitlyCreateCollection",
			"ns" : "test.bar"
		}
	},
	"ok" : 1,
	"operationTime" : Timestamp(1543854685, 3),
	"$clusterTime" : {
		"clusterTime" : Timestamp(1543854685, 5),
		"signature" : {
			"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
			"keyId" : NumberLong(0)
		}
	}
}

mongos> db.bar.getIndexes()
[
	{
		"v" : 2,
		"key" : {
			"_id" : 1
		},
		"name" : "_id_",
		"ns" : "test.bar"
	},
	{
		"v" : 2,
		"unique" : true,
		"key" : {
			"x" : 1
		},
		"name" : "x_1",
		"ns" : "test.bar"
	},
{
		"v" : 2,
		"unique" : true,
		"key" : {
			"_id" : 1,
			"x" : 1,
			"c" : 1
		},
		"name" : "_id_1_x_1_c_1",
		"ns" : "test.bar"
	}