'emerge -v --update --newuse --deep --with-bdeps=y --backtrack=30 @world' fails with: root@lynx2:/root(23)# emerge -v --update --newuse --deep --with-bdeps=y --backtrack=30 @world These are the packages that would be merged, in order: Calculating dependencies... done! [ebuild UD ] dev-util/boost-build-1.55.0-r1 [1.56.0] USE="examples python {-test}" PYTHON_TARGETS="python2_7" 56,784 KiB [ebuild UD ] dev-libs/boost-1.55.0-r2:0/1.55.0 [1.56.0-r1:0/1.56.0] USE="doc icu mpi nls python threads tools -context -debug -static-libs" ABI_X86="(64) -32 (-x32)" PYTHON_TARGETS="python2_7 python3_3 -python3_4" 0 KiB Total: 2 packages (2 downgrades), Size of downloads: 56,784 KiB !!! Multiple package instances within a single package slot have been pulled !!! into the dependency graph, resulting in a slot conflict: dev-libs/boost:0 (dev-libs/boost-1.56.0-r1:0/1.56.0::gentoo, installed) pulled in by dev-libs/boost:0/1.56.0= required by (sci-mathematics/singular-3.1.6-r1:0/0::sage-on-gentoo, installed) ^^^^^^^^^^ (and 17 more with the same problem) (dev-libs/boost-1.55.0-r2:0/1.55.0::gentoo, ebuild scheduled for merge) pulled in by dev-libs/boost:0/1.55.0= required by (sci-libs/libsingular-3.1.6-r4:0/0::sage-on-gentoo, installed) ^^^^^^^^^^ dev-util/boost-build:0 (dev-util/boost-build-1.56.0:0/0::gentoo, installed) pulled in by =dev-util/boost-build-1.56* required by (dev-libs/boost-1.56.0-r1:0/1.56.0::gentoo, installed) ^ ^^^^^ (dev-util/boost-build-1.55.0-r1:0/0::gentoo, ebuild scheduled for merge) pulled in by =dev-util/boost-build-1.55* required by (dev-libs/boost-1.55.0-r2:0/1.55.0::gentoo, ebuild scheduled for merge) ^ ^^^^^ root@lynx2:/root(24)# qlist -Iv singular sci-libs/libsingular-3.1.6-r4 sci-mathematics/singular-3.1.6-r1
That is curious (lib)singular are not specifying a boost version. However they have dev-libs/boost:0= which means a re-build should be triggered when you change the sub-slot. Which is what is done when you go from 1.55.0 to 1.56.0. Presumably un-merging singular and libsingular and then re-building them should fix the problem.