таки да, разрабы подтвердили что это windows specific баг G++.

объяснение звучит так:

The reason why g++ isn't able to distiguish between
calling-conventions for C++-function names is caused by the fact that
g++ doesn't use the calling-convention within mangled C++-name.
VC does this and so you have indeed two different signatures.

оформил баг-репорт.
жду подтверждения.