dev-cpp/eigen-3.0.5 requires the USE flags "latex" and "dot" to be enabled in app-doc/doxygen to successfully compile when using the eigen USE flag "doc".
I can't reproduce. Could you please post your emerge --info, and attach the full build log?
Created attachment 316551 [details] Output of emerge --info
If USE=dot is not set in doxygen the creation of the documentation for eigen-3.0.5 fails with the message: "Can't lstat eigen-doc/*.map" It appears that this is caused by Graphviz not generating .map files. Adding the USE=dot in doxygen fixed this issue for me. I attach the output of "emerge --info"; however, I don't have at hand the output of the full build log.
We need the build log to debug this issue further. Please reopen this bug when you are able to attach that.