Because there's no polymorphic_text_oarchive.cpp counterpart,
I think this is in the src directory. It should be pre-compiled into the library so you never actually see it. The portable binary archive is an example, so it's not included in the library.
There is no polymorphic_text_oarchive.cpp or polymorphic_text_iarchive.cpp
in the boost 1.42 source. A search in the source for
"polymorphic*archive.cpp" only reveals:
./libs/serialization/example/polymorphic_portable_binary_iarchive.cpp
./libs/serialization/example/polymorphic_portable_binary_oarchive.cpp
./libs/serialization/src/polymorphic_iarchive.cpp
./libs/serialization/src/polymorphic_oarchive.cpp
And the latter 2 files basically only have these explicitly instantiations:
template class archive_serializer_map