Improve Nullable<T> default checking

[ahdung]

I hope have a graceful way to check Nullable<T> is null or default(T).

// now
x.SubList == null || x.SubList.Count == 0
(x.SubList?.Count ?? 0) == 0
x.SubList?.Count is null or 0

// i hope
x.SubList?.Count is default // or some else like dualdefault | none | empty

[huoyaoyuan]

The current blocker is that you can't check if any struct is default in a general way.

[khm1600]

Structs don't have operator== defined by default.

[ahdung]

Interesting, this is different from my memory, wait me take a try

[ahdung]

you're right, i mistook (i almost never use custom structure).
But i don't think this is a insurmountable blocker, the complier can prevent structs which is unimplemented use ==, it also can prevent they use is default. partially usable still makes sense.

[yaakov-h]

.Equals is implemented by default.

https://sharplab.io/#v2:C4LgTgrgdgNAJiA1AHwAICYAMBYAUBgRjzwDcBDMAAgDMB7WygXkqgFMB3SgMXsoG9KAIQpNKmGELIAvUVAgAbeZQC+AbmL4CATgAUAEgBEdBgEsAzpTitqZBaH416AOgCiARwhl5ZnVZt2dHloASmCVA2D1XDwzYEgAY2Buejw+PEoMylQAZkoTKCThKgEAc1ZgVUozcsrlTPTMnKyCTEkZUpqqzuU8ZSA=

https://docs.microsoft.com/en-us/dotnet/api/system.valuetype.equals?view=net-5.0#remarks

[canton7]

It also boxes and uses reflection (in most cases). That might or might not matter.