Fix nullable detection

[rynowak]

Fixes: #11828 and #11813

[roji]

LGTM, two comments:

• Not sure if model processing speed is an issue, but you may want to look at caching context nullability at the type and module level, to avoid doing a lot of reflection over and over again. In EF Core we've seen some insane prolific models where this could be an issue.
• I can see the comment about not handling generics and NNRT, but we may want to consider identifying pre- and post-conditions such as [MaybeNull] (fully explained here).

[rynowak]

Thanks @roji

I'm not immediately concerned about intermediate caching, the results of this will all be cached as well.

I can see the comment about not handling generics and NNRT, but we may want to consider identifying pre- and post-conditions such as [MaybeNull]

Are these types actually defined anywhere? I keep seeing design notes, but I don't see these types in use anywhere.

[roji]

Are these types actually defined anywhere? I keep seeing design notes, but I don't see these types in use anywhere.

Take a look in corefx, they're used a lot there. BTW unlike NullableAttribute and NullableContextAttribute, these are actually defined in System.Diagnostics.CodeAnalysis (not sure why the different approach).

Note also https://github.com/dotnet/corefx/issues/36222#issuecomment-511973898 (things are going to change again a bit..).

[rynowak]

It seems like most of the overlap here is with generic-typed fields and properties right?

[roji]

I think so, but there are also some other cases. For example, you may want to have a property that can be set to null but will never return null (e.g. because it will return some sort of default). Exactly how we interpret that in ASP.NET/EF is also not immediately obvious.