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

Calculating dependencies ... done!
Dependency resolution took 2.24 s (backtrack: 8/20).

[ebuild  N     ] dev-lang/ocaml-5.5.1:0/5.5.1::gentoo  USE="ocamlopt -emacs -flambda -latex -test -xemacs" 6457 KiB
[ebuild  N     ] dev-lang/ocaml-4.14.2:0/4.14.2::gentoo  USE="ocamlopt -emacs -flambda -latex -xemacs" 5377 KiB
[ebuild  N     ] dev-ml/dune-3.23.1:0/3.23.1::gentoo  USE="-emacs -test" 3624 KiB
[ebuild  N     ] dev-ml/findlib-1.9.8-r1:0/1::gentoo  USE="ocamlopt -doc -tk" 269 KiB
[ebuild  N     ] dev-ml/sexplib0-0.17.0-r2:0/0.17::gentoo  USE="ocamlopt" 23 KiB
[ebuild  N     ] dev-ml/result-1.5:0/1.5::gentoo  USE="ocamlopt" 3 KiB
[ebuild  N     ] dev-ml/stdlib-shims-0.3.0::gentoo  USE="ocamlopt" 10 KiB
[ebuild  N     ] dev-ml/ocaml-compiler-libs-0.17.0:0/0.17.0::gentoo  USE="ocamlopt" 5 KiB
[ebuild  N     ] dev-ml/ppx_derivers-1.2.1:0/1.2.1::gentoo  USE="ocamlopt" 3 KiB
[ebuild  N     ] dev-ml/ocamlbuild-0.16.1:0/0.16.1::gentoo  USE="ocamlopt" 198 KiB
[ebuild  N     ] dev-ml/yojson-3.0.0:0/3.0.0::gentoo  USE="ocamlopt -examples -test" 56 KiB
[ebuild  N     ] dev-ml/ocaml_intrinsics_kernel-0.17.0:0/0.17::gentoo  USE="ocamlopt" 12 KiB
[ebuild  N     ] dev-ml/ppxlib-0.33.0-r2:0/0.33.0::gentoo  USE="ocamlopt -test" 624 KiB
[ebuild  N     ] dev-ml/topkg-1.1.1:0/1.1.1::gentoo  USE="ocamlopt" 119 KiB
[ebuild  N     ] dev-ml/csexp-1.5.2-r1:0/1.5.2::gentoo  USE="ocamlopt -test" 12 KiB
[ebuild  N     ] dev-ml/cmdliner-2.1.0:0/2.1.0::gentoo  USE="ocamlopt" 96 KiB
[ebuild  N     ] dev-ml/dune-configurator-3.21.1:0/3.21.1::gentoo  USE="ocamlopt" 3352 KiB
[ebuild  N     ] dev-ml/base-0.17.3:0/0.17.3::gentoo  USE="ocamlopt" 468 KiB
[ebuild  N     ] dev-ml/ppxlib-0.39.0:0/0.39.0::gentoo  USE="ocamlopt -test" 687 KiB
[ebuild  N     ] dev-ml/ppxlib_jane-0.17.4-r1:0/0.17::gentoo  USE="ocamlopt" 42 KiB
[ebuild  N     ] dev-ml/ppx_sexp_conv-0.17.0-r2:0/0.17::gentoo  USE="ocamlopt" 67 KiB

Total: 21 packages (21 new), Size of downloads: 21494 KiB

!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:

dev-ml/ppxlib:0

  (dev-ml/ppxlib-0.33.0-r2:0/0.33.0::gentoo, ebuild scheduled for merge) USE="ocamlopt -test" pulled in by
    <dev-ml/ppxlib-0.36.0 required by (dev-ml/ppx_sexp_conv-0.17.0-r2:0/0.17::gentoo, ebuild scheduled for merge) USE="ocamlopt"
    ^              ^^^^^^                                                                                                                                                                                 

  (dev-ml/ppxlib-0.39.0:0/0.39.0::gentoo, ebuild scheduled for merge) USE="ocamlopt -test" pulled in by
    >=dev-ml/ppxlib-0.36.0:=[ocamlopt?] required by (dev-ml/ppxlib_jane-0.17.4-r1:0/0.17::gentoo, ebuild scheduled for merge) USE="ocamlopt"
    ^^              ^^^^^^                                                                                                                                                                                                               

dev-lang/ocaml:0

  (dev-lang/ocaml-5.5.1:0/5.5.1::gentoo, ebuild scheduled for merge) USE="ocamlopt -emacs -flambda -latex -test -xemacs" pulled in by
    >=dev-lang/ocaml-5.3 required by (dev-ml/ppxlib_jane-0.17.4-r1:0/0.17::gentoo, ebuild scheduled for merge) USE="ocamlopt"
    ^^               ^^^                                                                                                                                                                               
    (and 4 more with the same problem)

  (dev-lang/ocaml-4.14.2:0/4.14.2::gentoo, ebuild scheduled for merge) USE="ocamlopt -emacs -flambda -latex -xemacs" pulled in by
    <dev-lang/ocaml-5.3:= required by (dev-ml/ppxlib-0.33.0-r2:0/0.33.0::gentoo, ebuild scheduled for merge) USE="ocamlopt -test"
    ^               ^^^ ^                                                                                                                                                                                                                        

NOTE: Use the '--verbose-conflicts' option to display parents omitted above

It may be possible to solve this problem by using package.mask to
prevent one of those packages from being selected. However, it is also
possible that conflicting dependencies exist such that they are
impossible to satisfy simultaneously.  If such a conflict exists in
the dependencies of two different packages, then those packages can
not be installed simultaneously. You may want to try a larger value of
the --backtrack option, such as --backtrack=30, in order to see if
that will solve this conflict automatically.

For more information, see MASKED PACKAGES section in the emerge man
page or refer to the Gentoo Handbook.