using System;

public class A
{
    
static void NotNull<T>(T obj)
    where T : class
{
}

static void NotNull<T>(T? obj)
    where T : struct
{
}
    
    static void F<T>(T t) { NotNull(t); }
    
    public static void Main()
    {
    }
}

Compilation error (line 16, col 26): The type 'T' must be a reference type in order to use it as parameter 'T' in the generic type or method 'A.NotNull<T>(T)