Hi Beman, hi all Due to recent changes to the filesystem library now, the header detail/identifier.hpp is now included in my projects and contains the dangerous code segment: template <class Ostream, class Id> typename enable_if< is_base_of< identifier< typename Id::value_type, Id >, Id >, Ostream & >::type operator<<( Ostream & os, const Id & id ) { return os << id.value(); } which breaks all of my codes, since I have a type Id, that has a type member Id::value_type which is an abstract base class. My compiler (Apple gcc-4.0) now tries to instantiate identifier< typename Id::value_type, Id >, which fails because Id::value_type is abstract, and the compiler aborts with an error message. This causes all of our codes to fail to compile with the current CVS head Matthias