Free Software Foundation Compatibility?
Public Domain and Permissively Licensed (MIT/BSD) code can also be considered "GPL code" when it's incorporated into GPL projects.
Fair-Code Licensed code cannot automatically be considered "GPL code"
The Free Software Foundation (FSF) is an activist organization for the promotion of the GPL.
The FSF not advocate for the Open Source Initiative (OSI) and "Open Source" because it misses the point of free-software.
The FSF wrote Avoiding Ruinous Compromise concerning their goal to "make all software free".
FSF founder Richard Stallman's thoughts are clearly captured in host post clang vs free software
The existence of LLVM is a terrible setback for our community precisely because it is not copylefted and can be used as the basis for nonfree compilers
Any software that is not explicitly GPL licensed will incur some amount of dissatisfaction on the part of the FSF.