NotNullIfNotNull is not recognized inside a generic method body.

[manfred-brands]

Version Used:
VS 2019 16.6.2
C# 8 with nullability enabled.

Steps to Reproduce:

        [return: NotNullIfNotNull("defaultValue")]
        private static T Find<T>([AllowNull] T defaultValue)
        {
            return defaultValue; // CS8603
        }

        [return: NotNullIfNotNull("defaultValue")]
        private static T FindOrCreate<T>([AllowNull] T defaultValue)
        {
            return Find(defaultValue);
        }

Expected Behavior:

No CS8603 raised at the return defaultValue statement.

Actual Behavior:

CS8603 is raised at the return defaultValue statement.
Adding [return: MaybeNull] removes the CS8603 in the first function, but then raises it in the second.
It is related to 36039 which fixed it for MaybeNull.

[RikkiGibson]

My interpretation here is that while you have [return: NotNullIfNotNull], these methods are still not permitted to return a maybe-null value when T is a non-nullable reference type.

There are two possible solutions to this:

1. You could add [return: MaybeNull] to both methods, since as written, both might return a maybe-null value when T is a non-nullable reference type.

2. You could delete the [AllowNull] attribute from the parameters, which would make it so maybe-null values are only acceptable when T is a nullable reference type.

[manfred-brands]

With C#9 I can actually declared the return as T? solving the problem.