I have a package which currently depends on gfortran-5, although
gfortran-5 doesn't build on core-updates.

efraimf@penguin2:~/workspace/guix-core-updates$ ./pre-inst-env guix build --no-grafts -e '(@@ (gnu packages gcc) gfortran-5)'
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
The following derivation will be built:
   /gnu/store/frqb5p2vdk8qcjwbny1s6mlak344a0gx-gfortran-5.5.0.drv
building /gnu/store/frqb5p2vdk8qcjwbny1s6mlak344a0gx-gfortran-5.5.0.drv...
Backtrace:
In ice-9/eval.scm:
   217:50 19 (lp (#<procedure 7fffee835f40 at ice-9/eval.scm:282:?> ?))
   217:50 18 (lp (#<procedure 7fffee835f20 at ice-9/eval.scm:282:?> ?))
   217:50 17 (lp (#<procedure 7fffee835f00 at ice-9/eval.scm:282:?> ?))
   217:50 16 (lp (#<procedure 7fffee835ee0 at ice-9/eval.scm:282:?> ?))
   217:50 15 (lp (#<procedure 7fffee835e80 at ice-9/eval.scm:649:?> ?))
   217:50 14 (lp (#<procedure 7fffee835e60 at ice-9/eval.scm:282:?> ?))
   217:50 13 (lp (#<procedure 7fffee835e00 at ice-9/eval.scm:649:?> ?))
   217:50 12 (lp (#<procedure 7fffee835de0 at ice-9/eval.scm:282:?> ?))
   217:50 11 (lp (#<procedure 7fffee835dc0 at ice-9/eval.scm:282:?> ?))
   217:50 10 (lp (#<procedure 7fffee835da0 at ice-9/eval.scm:282:?> ?))
   217:50  9 (lp (#<procedure 7fffee835b20 at ice-9/eval.scm:649:?> ?))
   217:50  8 (lp (#<procedure 7fffee835b00 at ice-9/eval.scm:282:?> ?))
   217:50  7 (lp (#<procedure 7fffee835ae0 at ice-9/eval.scm:282:?> ?))
   217:50  6 (lp (#<procedure 7fffee835ac0 at ice-9/eval.scm:282:?> ?))
   217:33  5 (lp (#<procedure 7fffee7fe9c0 at ice-9/eval.scm:196:?> ?))
   196:43  4 (_ #f)
   196:35  3 (_ #f)
   202:27  2 (_ #f)
   223:20  1 (proc #<directory (guile-user) 7fffefbadf00>)
   In unknown file:
           0 (%resolve-variable (7 . cut) #<directory (guile-user) 7?>)

ERROR: In procedure %resolve-variable:
Unbound variable: cut
builder for `/gnu/store/frqb5p2vdk8qcjwbny1s6mlak344a0gx-gfortran-5.5.0.drv' failed with exit code 1
build of /gnu/store/frqb5p2vdk8qcjwbny1s6mlak344a0gx-gfortran-5.5.0.drv failed
View build log at '/var/log/guix/drvs/fr/qb5p2vdk8qcjwbny1s6mlak344a0gx-gfortran-5.5.0.drv.bz2'.
guix build: error: build of `/gnu/store/frqb5p2vdk8qcjwbny1s6mlak344a0gx-gfortran-5.5.0.drv' failed

I checked the guix tree and 'git grep \,gfortran' showed they're all
using gfortran-7. I was able to build gfortran-6. gfortran-4.9 also
failed, same error.

-- 
Efraim Flashner   <efraim@flashner.co.il>   אפרים פלשנר
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