--conservative is not working.

gem always tries to upgrade the dependencies of the to-be-installed package even with the --conservative flag set. In my case I'm trying to avoid updating a gem installed (and managed) by the host OS (Gentoo).
In this e.g. ostruct and bigdecimal gems are already installed and part of the ruby installation --

gem search -d -l ostruct

*** LOCAL GEMS ***

ostruct (0.5.5)
    Author: Marc-Andre Lafortune
    Homepage: https://github.com/ruby/ostruct
    Licenses: Ruby, BSD-2-Clause
    Installed at (default): /usr/lib64/ruby/gems/3.2.0

    Class to build custom data structures, similar to a Hash.

gem search -d -l bigdecimal

*** LOCAL GEMS ***

bigdecimal (3.1.3)
    Authors: Kenta Murata, Zachary Scott, Shigeo Kobayashi
    Homepage: https://github.com/ruby/bigdecimal
    Licenses: Ruby, bsd-2-clause
    Installed at (default): /usr/lib64/ruby/gems/3.2.0

    Arbitrary-precision decimal floating-point number library.

I'm trying to install the 'oj' gem --

gem dependency -r -v 3.16.9 oj
Gem oj-3.16.9
  bigdecimal (>= 3.0)
  minitest (~> 5, development)
  ostruct (>= 0.2)
  rake-compiler (>= 0.9, < 2.0, development)
  test-unit (~> 3.0, development)

The currently installed bigdecimal and ostruct versions of the gem are good enough to satisfy the dependencies. However --

gem install --conservative --minimal-deps --explain -v 3.16.9 oj
Gems to install:
  ostruct-0.6.1
  bigdecimal-3.1.9
  oj-3.16.9

RubyGems Environment:
  - RUBYGEMS VERSION: 3.4.22
  - RUBY VERSION: 3.2.5 (2024-07-26 patchlevel 208) [x86_64-linux]
  - INSTALLATION DIRECTORY: /usr/lib64/ruby/gems/3.2.0
  - USER INSTALLATION DIRECTORY: /home/de/.local/share/gem/ruby/3.2.0
  - RUBY EXECUTABLE: /usr/bin/ruby32
  - GIT EXECUTABLE: /usr/bin/git
  - EXECUTABLE DIRECTORY: /usr/bin
  - SPEC CACHE DIRECTORY: /home/de/.local/share/gem/specs
  - SYSTEM CONFIGURATION DIRECTORY: /etc
  - RUBYGEMS PLATFORMS:
     - ruby
     - x86_64-linux
  - GEM PATHS:
     - /usr/lib64/ruby/gems/3.2.0
     - /home/de/.local/share/gem/ruby/3.2.0
     - /usr/local/lib64/ruby/gems/3.2.0
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => true
     - :backtrace => true
     - :bulk_threshold => 1000
     - "install" => "--install-dir /home/de/.local/share/gem/ruby/3.2.0 --bindir /home/de/bin"
     - "uninstall" => "--install-dir /home/de/.local/share/gem/ruby/3.2.0 --bindir /home/de/bin"
     - "update" => "--install-dir /home/de/.local/share/gem/ruby/3.2.0 --bindir /home/de/bin"
  - REMOTE SOURCES:
     - https://rubygems.org/
  - SHELL PATH:
     - /usr/local/sbin
     - /usr/local/bin
     - /usr/bin
     - /opt/bi

If I gem install --ignore-dependencies oj oj works as expected since the dependencies are already complete.

Start by reproducing the dependency resolution shown by gem install --conservative --minimal-deps --explain -v 3.16.9 oj, comparing it with --ignore-dependencies. Trace the gem install handling of --conservative and --minimal-deps through dependency resolution. Done means already-installed, compatible ostruct and bigdecimal gems are retained while installing oj.