Default argument expressions must be valid for all type arguments. This means they can only be defined in terms of API made visible by the constraints on the corresponding type argument.
I pitched [Pre-pitch] Conditional default arguments a while ago which would allow you to specify this default conditionally. That pitch is syntactic sugar to avoid needing to write overloads that omit the type argument and corresponding method argument. Short of that feature being implemented, it’s always possible to write the overloads manually, even if it is not ideal.