Roslyn allows additional *primary_constraints* when the notnull primary constraint is used

[BillWagner]

Version Used:

C# 12 and C# 13, with LangVersion set from C# 8 through C# 13.

Use the following code:

using System.IO;

public class C1<T> where T : notnull, Stream {}
public class C2<T> where T : notnull, Stream? {}

See sharplab

Expected Behavior:

According to 15.2.5 only one primary_constraint can be specified. The Nullable reference types specification - 9.0 classifies the notnull constraint as a primary constraint. Therefore, both the above declarations should be prohibited.

Actual Behavior:

Both declarations are allowed.

See dotnet/csharpstandard#1178 (comment)

[jaredpar]

@jcouv, @RikkiGibson is this a spec or impl bug?

[RikkiGibson]

This appears to be an impl bug as the csharplang spec works by adding notnull as a new production of primary_constraint.

However, as mentioned on the csharpstandard PR, I wouldn't expect us to make a breaking change here, except perhaps adding a warning in the where T : notnull, Class or similar cases to indicate that notnull is notnecessary.