Sorry for the wordy description, but I thought it was necessary to justify my request...
I was having trouble compiling LibreOffice with clang. I found bug 760549 which exactly described my build error, but it's marked as RESOLVED FIXED, which is strange... It also has at least two duplicates.
I read through that bug's comments and the page for clang in the wiki, and I discovered that enabling the default-libcxx USEflag for clang on a system set up to use GCC as the primary compiler is actually a configuration error, because libstdc++ and libc++ are not ABI compatible and the majority of C++ packages one attempts to build with clang will fail to link.
It's conceivable that a user might change this USEflag while experimenting with clang on their system and think it's normal for some pacakges not to compile when using a different compiler. Personally I assumed it was a problem with LibreOffice, because Firefox and Thunderbird also have clang USEflags and are able to build just fine, so I didn't think it could be my own fault. It's also possible that bugs for other packages have been opened because of this USEflag, but I happened to run into the problem with LibreOffice.
Therefore, I think it would be beneficial if sys-devel/clang would display a warning in postinst if the default-libcxx USEflag is enabled and it detects that GCC is the default compiler. This could potentially reduce the number of bugs opened in the tracker that turn out to be caused by configuration errors.
I was indeed able to build LibreOffice with clang after changing the USEflag.
Steps to Reproduce:
1. emerge sys-devel/clang with default-libcxx USEflag on an otherwise mostly sane ~amd64 system
2. (attempt to) emerge app-office/libreoffice
See bug 760549.
Some package along the line (sys-devel/clang seems like the most proper place) should be able to detect this configuration error and warn the user that things are going to break. Then the user can either change the USEflag or rebuild all C++ packages against libc++ so packages will compile as expected.
Due to the nature of this bug/request I don't believe it's relevant to share my "emerge --info".