Why is this an issue?

When constraints have not been applied to restrict a generic type parameter to be a reference type, then a value type, such as a struct, could also be passed. In such cases, comparing the type parameter to null would always be false, because a struct can be empty, but never null. If a value type is truly what’s expected, then the comparison should use default(). If it’s not, then constraints should be added so that no value type can be passed.

Noncompliant code example

private bool IsDefault<T>(T value)
{
  if (value == null) // Noncompliant
  {
    // ...
  }
  // ...
}

Compliant solution

private bool IsDefault<T>(T value)
{
  if(object.Equals(value, default(T)))
  {
    // ...
  }
  // ...
}

or

private bool IsDefault<T>(T value) where T : class
{
  if (value == null)
  {
    // ...
  }
  // ...
}