Casting for top-level $expr

[vkarpov15]

Do you want to request a feature or report a bug?

Feature

What is the current behavior?

Re: #10622, right now we don't do any casting on $expr. And sanitizeFilter doesn't do anything with $expr either.

If the current behavior is a bug, please provide the steps to reproduce.

const mongoose = require('mongoose');
mongoose.set('debug', true)
const testSchema = new mongoose.Schema({ date: {type: Date, required: true }}, {timestamps: {createdAt: 'created_at', updatedAt: 'updated_at'}})

testSchema.index({ date: -1});

const Test = mongoose.model('Test', testSchema);

async function test() {
    await mongoose.connect('mongodb://localhost:27017/test', {
        useNewUrlParser: true,
        useUnifiedTopology: true,
      });
      await mongoose.connection.dropDatabase();

      await Test.find( { $expr: { $eq: [{ $year: '$date' }, 2021] } }, {date: 1}).sort({ date: -1 });
      console.log('Done');
}

test();

What is the expected behavior?

Not sure yet, but something we need to figure out. Questions:

1. Should sanitizeFilter() remove $expr?

2. How much casting can we reasonably do on $expr? In the above example, can we figure out that 2021 needs to be a number?

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.