$ spack -e core concretize
==> Error: Conflicts in concretized spec "clingo-bootstrap@spack%gcc@4.8.5~docs~ipo+python build_type=Release arch=linux-centos7-x86_64/oeui676"
List of matching conflicts for spec:
    clingo-bootstrap@spack%gcc@4.8.5~docs~ipo+python build_type=Release arch=linux-centos7-x86_64
        ^bison@3.7.6%gcc@4.8.5 arch=linux-centos7-x86_64
            ^diffutils@3.7%gcc@4.8.5 arch=linux-centos7-x86_64
                ^libiconv@1.16%gcc@4.8.5 arch=linux-centos7-x86_64
            ^m4@1.4.19%gcc@4.8.5+sigsegv arch=linux-centos7-x86_64
                ^libsigsegv@2.13%gcc@4.8.5 arch=linux-centos7-x86_64
            ^perl@5.34.0%gcc@4.8.5+cpanm+shared+threads arch=linux-centos7-x86_64
                ^berkeley-db@18.1.40%gcc@4.8.5+cxx~docs+stl patches=b231fcc4d5cff05e5c3a4814f6a5af0e9a966428dc2176540d2c05aff41de522 arch=linux-centos7-x86_64
                ^gdbm@1.19%gcc@4.8.5 arch=linux-centos7-x86_64
                    ^readline@8.1%gcc@4.8.5 arch=linux-centos7-x86_64
                        ^ncurses@6.2%gcc@4.8.5~symlinks+termlib abi=none arch=linux-centos7-x86_64
                            ^pkgconf@1.7.4%gcc@4.8.5 arch=linux-centos7-x86_64
        ^cmake@3.20.3%gcc@4.8.5~doc+ncurses+openssl+ownlibs~qt build_type=Release arch=linux-centos7-x86_64
            ^openssl@1.1.1k%gcc@4.8.5~docs+systemcerts arch=linux-centos7-x86_64
                ^zlib@1.2.11%gcc@4.8.5+optimize+pic+shared arch=linux-centos7-x86_64
        ^python@3.6%gcc@4.8.5+bz2+ctypes+dbm~debug+libxml2+lzma~nis~optimizations+pic+pyexpat+pythoncmd+readline+shared+sqlite3+ssl~tix~tkinter~ucs4+uuid+zlib arch=linux-centos7-x86_64
        ^re2c@1.2.1%gcc@4.8.5 arch=linux-centos7-x86_64
1. "%gcc@:5.99.99" conflicts with "clingo-bootstrap" [C++14 support is required to bootstrap clingo]