The standard does not address implementation details. There are many,
many things which depend on the implementation, and which prevent
programs from working together: how the classes are laid out, the
structure of the vtable, etc. In general, compilers will change the
name mangling if they change any of these. This is intentional, as it
prevents code which would not work from linking.
It is possible for a given platform to define a C++ ABI; all compilers
which adhere to it would use compatible implementations, and have a
common name mangling. This is an issue for the platform vendors,
however; for whatever reasons, very few vendors have defined a C++ ABI.
And the reason extern "C" works is because almost all platforms define a C ABI.