Custom Equality check for C# 9 records

Per the C#9 record proposal, the following should compile, even if not very useful without actual implementations..

// No explicit IEquatable<R> - this is synthesized!
public sealed record SimpleVo
{
    // Not virtual, as SimpleVo (R) is sealed.
    // Accepts SimpleVo? (R?), and not SimpleVo (R), as argument.
    public bool Equals(SimpleVo? other) =>
        throw new System.NotImplementedException();

    // Optional: warning generated if not supplied when Equals(R?) is user-defined.
    public int GetHashCode() =>
        throw new System.NotImplementedException();

    // No other “standard” equality members!
}

There are restrictions on the equality-related members as most of the code is synthesized. The proposal includes examples of the expected synthesized underlying type.

That is, given just a Equals(R?) the compiler creates a ==, !=, and Equals(object).
The methods that can be defined can be found by searching for “user-defined” in the proposal.

Attempting to override/define other equality methods or operators is expected to fail:

It is an error if the override is declared explicitly.

The behavior is discussed in ‘Equality members’ and is summarized in the paragraph:

The record type implements System.IEquatable<R> and includes a synthesized strongly-typed overload of book Equals(R? other) where R is the record type. The method is public, and the method is virtual unless the record type is sealed. The [Equals(R?)] method can be declared explicitly. It is an error if the explicit declaration does not match the expected signature or accessibility, or the explicit declaration doesn’t allow overriding it in a derived type and the record type is not sealed.
If Equals(R? other) is user-defined (not synthesized) but GetHashCode is not [user-defined], a warning is produced.