These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 1.54 s (backtrack: 0/20).

[ebuild     U  ] dev-cpp/eigen-3.4.1-r1:3/3.4::gentoo [3.4.0-r3:3/3::gentoo] USE="openmp -adolc (-blas) -boost -cholmod -cuda -cuda-clang% -debug (-doc) -fftw -hip -klu (-lapack) -mathjax (-metis) (-opengl*) (-pastix) -pocketfft% -sparsehash -spqr -superlu (-test) -umfpack (-clang%)" CPU_FLAGS_X86="avx avx2 avx512dq avx512f f16c fma3 popcnt sse sse2 sse3 sse4_1 sse4_2 ssse3" LLVM_SLOT="22%* -21% -23%" 2109 KiB

Total: 1 package (1 upgrade), Size of downloads: 2109 KiB