Version Used:

Steps to Reproduce:

Input this code

class Program
{
    void M()
    {
        string numberString = "One";
        Numbers? number = numberString switch
        {
            "One" => (Numbers?)Numbers.One,
            "Two" => Numbers.Two,
            _ => null,
        };
    }
}

enum Numbers
{
    One,
    Two
}

The cast (Numbers?) in case "One" is marked as unnecessary.

Removing the cast causes a compiler error.
Error CS0037 Cannot convert null to 'Numbers' because it is a non-nullable value type

A few followup notes

1. It is a little confusing that casting the first element of the switch expression is required, even if I explicitly define the expected return type as Numbers?. The IDE also offers no assistance that casting the first member of the switch expression may fix the compiler error.
2. Converting a switch statement to switch expression in this instance yields invalid code (it doesn't add the appropriate cast to the first case).