Symfony version(s) affected: 4.4

Description
When using inheritance, if you have multi-level inheritance, you have to set the DiscriminatorMap in the correct order for the Serializer to find the right class because of the way it tests the class type.

How to reproduce
Create 2 classes to serialize with inheritance:
Foo
Bar extends Foo

Set the DiscriminatorMap like this:

@DiscriminatorMap(typeProperty="type", mapping={
 *   "foo"    = "App\Foo",
 *   "bar"   = "App\Bar"
 * })

If you serialize this, your Bar class will have to "type" set to Foo.

Now, invert the map:

@DiscriminatorMap(typeProperty="type", mapping={
 *   "bar"   = "App\Bar",
 *   "foo"    = "App\Foo"
 * })

The Bar class will have the correct type.

This is because the function that checks the type uses is_a() and then exits the mapping loop if one returns true.

public function getMappedObjectType($object): ?string
    {
        foreach ($this->typesMapping as $type => $typeClass) {
            if (is_a($object, $typeClass)) {
                return $type;
            }
        }

        return null;
    }

Possible Solution
Improve the loop detecting the type and check precedence