public abstract partial class A
{
    public abstract TResult? Accept<TResult>(int a);
}

public sealed partial class B : A
{
    public override TResult? Accept<TResult>(int a) where TResult : default { throw null; }
}

public abstract partial class A
{
    public abstract TResult? Accept<TResult>(int a);
}
public sealed partial class B : A
{
    /// error CS0508: 'B.Accept<TResult>(int)': return type must be 'TResult' to match overridden member 'A.Accept<TResult>(int)'
    /// error CS0453: The type 'TResult' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'Nullable<T>'
    public override TResult? Accept<TResult>(int a) { throw null; }
}