NBMsgPack073: UseComparerAttribute type must not be abstract unless using static member

UseComparerAttribute should not specify an abstract type unless a static member is also specified to provide the actual comparer instance.

Example violation

[GenerateShape]
public partial class MyType
{
    [UseComparer(typeof(AbstractComparer))]
    public HashSet<string> MyHashSet { get; set; } = new();
}

public abstract class AbstractComparer : IEqualityComparer<string>
{
    public abstract bool Equals(string? x, string? y);
    public abstract int GetHashCode(string obj);
}

Resolution

Either use a concrete type or specify a static member on the abstract type:

[GenerateShape]
public partial class MyType
{
    // Option 1: Use a concrete type
    [UseComparer(typeof(ConcreteComparer))]
    public HashSet<string> MyHashSet1 { get; set; } = new(new ConcreteComparer());

    // Option 2: Use a static member from an abstract type
    [UseComparer(typeof(AbstractComparerProvider), nameof(AbstractComparerProvider.Default))]
    public HashSet<string> MyHashSet2 { get; set; } = new(AbstractComparerProvider.Default);
}

public abstract class AbstractComparerProvider
{
    public static IEqualityComparer<string> Default => StringComparer.OrdinalIgnoreCase;
}

public class ConcreteComparer : IEqualityComparer<string>
{
    public bool Equals(string? x, string? y) => string.Equals(x, y, StringComparison.OrdinalIgnoreCase);
    public int GetHashCode(string obj) => obj?.ToUpperInvariant().GetHashCode() ?? 0;
}