Can Clang compile code with GCC compiled .a libs?

At least for Crypto++ library this does not work (verified :-( ). So for c++ code it is less likely to work, while pure c code would probably link OK.

EDIT: The problem started appearing with Mac OS X 10.9 Mavericks and Xcode-5, which switched the default C++ library for clang from libstdc++ to libc++. It did not exist on Mac OS X 10.8 and earlier.

The solution appears to be: if you need to compile C++ code with clang, and link it to a gcc-compiled library, use "clang++ -stdlib=libstdc++". The linking is successful, and the resulting binary runs correctly.

CAVEAT: It does not seem to work the other way: even though you can build a library compiled with "clang++ -stdlib=libstdc++" and link gcc-compiled code with it, this code will crash with SEGV. So far I found the only way to link with a clang-compiled library is compiling your code with clang, not gcc.

EDIT2: GCC-12 seems to include -stdlib= flag. Compiling with g++ -stdlib=libc++ creates Clang++-compatible object files. Very nice.

Yes, you usually can use clang with GCC compiled libraries (and vice versa, use gcc with CLANG compiled libraries), because in fact it is not compilation but linking which is relevant. You might be unlucky and get unpleasant suprises.

You could in principle have some dependencies on the version of libstdc++ used to link the relevant libraries (if they are coded in C++). Actually, that usually does not matter much.

In C++, name mangling might in theory be an issue (there might be some corner cases, even incompatibilities between two different versions of g++). Again, in practice it is usually not an issue.

So usually you can mix CLANG (even different but close versions of it) with GCC but you may have unpleasant surprises. What should be expected from any C++ compiler (be it CLANG or GCC) is just to be able to compile and link an entire software (and all libraries) together using the same compiler and version (and that includes the same C++ standard library implementation). This is why upgrading a compiler in a distribution is a lot of work: the distribution makers have to ensure that all the packages compile well (and they do get surprises!).

Beware that the version of libstdc++ does matter. Both Clang & GCC communities work hard to make its ABI compatible for compiler upgrades, but there are subtle corner cases. Read the documentation of your particular and specific C++ standard library implementation. These corner cases could explain mysterious crashes when using a good C++ library binary (compiled with GCC 5) in your code compiled with GCC 8. The bug is not in the library, but the ABI evolved incompatibly.