t = db.update_arraymatch4;
t.drop();

x = {
    _id: 1,
    arr: ["A1", "B1", "C1"]
};
t.insert(x);
assert.eq(x, t.findOne(), "A1");

x.arr[0] = "A2";
t.update({arr: "A1"}, {$set: {"arr.$": "A2"}});
assert.eq(x, t.findOne(), "A2");

t.ensureIndex({arr: 1});
x.arr[0] = "A3";
t.update({arr: "A2"}, {$set: {"arr.$": "A3"}});
assert.eq(x, t.findOne(), "A3");  // SERVER-1055