http://bugs.winehq.org/show_bug.cgi?id=657
--- Comment #64 from Jerome Leclanche adys.wh@gmail.com 2010-09-03 23:48:09 CDT --- (In reply to comment #63)
Just because it is publicly available does not mean it's not licensed such that a derivative work is illegal.
IANAL, but it sounds like there is a difference between "writing derivative work", and plain old "related, non-derivative work after looking at publicly available source code".
It also sounds like "having looked at private microsoft source code" most likely means you're an ex-microsoft employee or partner, which wouldn't apply to MFC42.
Once again, not a lawyer, but it's worth doublechecking.