Verify older compilers consuming newer library that uses the new constraints

[jcouv]

Ported issue from #22152 (comment)

[RikkiGibson]

Does this refer specifically to the 'notnull' constraint? Or to other features as well?

[jcouv]

class?, class (in a #nullable enable context) and notnull

[RikkiGibson]

I created a small project to manually test this. As expected, the project using Roslyn 2.x ignored the notnull constraint and interpreted the class constraints inside '#nullable enable' context the same as oblivious class constraints would be interpreted.

ClassLibrary

(using Roslyn 3.3.x compiler, LangVersion preview, targeting netstandard2.0)

public class Class1<T> where T : notnull
{
    public void M(T t) { }
}

public class Class2<T> where T : class
{
    public void M(T t) { }
}

public class Class3<T> where T : class?
{
    public void M(T t) { }
}

public class Class4
{
    public static void M1<T>(T t) where T : notnull { }
    public static void M2<T>(T t) where T : class { }
    public static void M3<T>(T t) where T : class? { }
}

ConsoleApp

(using Roslyn 2.10.x compiler, default LangVersion, targeting netcoreapp2.2)

class Program
{
    static void Main(string[] args)
    {
        new Class1<string>().M(null);
        new Class1<string>().M("a");

        new Class1<int?>().M(null);
        new Class1<int?>().M(42);
        new Class1<int>().M(42);

        new Class2<string>().M(null);
        new Class2<string>().M("a");

        new Class3<string>().M(null);
        new Class3<string>().M("a");

        Class4.M1((string)null);
        Class4.M1("a");

        Class4.M2((string)null);
        Class4.M2("a");

        Class4.M3((string)null);
        Class4.M3("a");

        // the non-nullable 'class' and 'class?' constraint
        // is correctly seen as just a class constraint
        // new Class2<int>();
        // new Class3<int>();
        // Class4.M2(42);
        // Class4.M3(42);

        Console.WriteLine("done");
        Console.ReadKey();
    }
}