This is an auto-filled bug because sci-libs/arpack calls commands that do not exist. The issue was originally discovered on arm64, but it may be reproducible on other arches as well. If you think that a different summary clarifies the issue better, feel free to change it. Attached build log and emerge --info.
Created attachment 636600 [details] build.log build log and emerge --info
Took me 5 minutes to figure it out. The ebuild is missing a "\" at the end of line 37. https://gitweb.gentoo.org/repo/gentoo.git/tree/sci-libs/arpack/arpack-3.5.0.ebuild#n37 This is really showing off in the way configure is called. ./configure --prefix=/usr --build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu --mandir=/usr/share/man --infodir=/usr/share/info --datadir=/usr/share --sysconfdir=/etc --localstatedir=/var/lib --disable-dependency-tracking --disable-silent-rules --docdir=/usr/share/doc/arpack-3.5.0 --htmldir=/usr/share/doc/arpack-3.5.0/html --with-sysroot=/ --libdir=/usr/lib64 --disable-static No --with-blas=..., no --with-lapack=... and no --{en,dis}able-mpi at the end. That's because bash tries to execute that as a separate command since it is not part of the configure line.
Fixed in https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=f54c1486bd1db164892b22b447019866346f0faf by grknight.
*** Bug 722272 has been marked as a duplicate of this bug. ***
