Script to reproduce the behavior described in https://github.com/rubygems/rubygems/issues/3708

repro-3708.sh

#!/bin/bash

# I run this only on macOS

set -ex

ISSUE=3708
ROOT_DIR=/tmp/repro-$ISSUE

rm -rf $ROOT_DIR
mkdir -p $ROOT_DIR
cd $ROOT_DIR

bundle env
# ## Environment
#
# ```
# Bundler       2.1.4
#   Platforms   ruby, x86_64-darwin-19
# Ruby          2.7.1p83 (2020-03-31 revision a0c7c23c9cec0d0ffcba012279cd652d28ad5bf3) [x86_64-darwin19]
#   Full Path   /Users/gio/.rbenv/versions/2.7.1/bin/ruby
#   Config Dir  /Users/gio/.rbenv/versions/2.7.1/etc
# RubyGems      3.1.2
#   Gem Home    /Users/gio/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0
#   Gem Path    /Users/gio/.gem/ruby/2.7.0:/Users/gio/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0
#   User Home   /Users/gio
#   User Path   /Users/gio/.gem/ruby/2.7.0
#   Bin Dir     /Users/gio/.rbenv/versions/2.7.1/bin
# Tools
#   Git         2.26.2
#   RVM         not installed
#   rbenv       rbenv 1.1.2
#   chruby      not installed
# ```
#
# ## Bundler Build Metadata
#
# ```
# Built At          2020-06-11
# Git SHA           unknown
# Released Version  false
# ```

cat > Gemfile <<RUBY
source 'https://rubygems.org'

# Note: these gems are chosen purely because they come with no dependencies.
# The behavior can be reproduced with any gem set.

gem 'dotenv'

group :optional_group, optional: true do
  gem 'safe_yaml'
end
RUBY

bundle config set --local path "vendor/bundle"

cat .bundle/config
# ---
# BUNDLE_PATH: "vendor/bundle

bundle install
# Fetching gem metadata from https://rubygems.org/..
# Resolving dependencies...
# Using bundler 2.1.4
# Fetching dotenv 2.7.5
# Installing dotenv 2.7.5
# Bundle complete! 2 Gemfile dependencies, 2 gems now installed.
# Bundled gems are installed into `./vendor/bundle`
#
# ✅ this is what I'd expect only dotenv installed, in the specified path.

BUNDLE_WITH=optional_group bundle install
# Fetching gem metadata from https://rubygems.org/.........
# Using bundler 2.1.4
# Using dotenv 2.7.5
# Fetching safe_yaml 1.0.5
# Installing safe_yaml 1.0.5
# Bundle complete! 2 Gemfile dependencies, 3 gems now installed.
# Bundled gems are installed into `./vendor/bundle`
#
# ✅ this is what I'd expect all gems installed, in the specified path

cat .bundle/config
# ---
# BUNDLE_PATH: "vendor/bundle"
# BUNDLE_WITH: "optional_group"
#
# 🤔 I wasn't expecting .bundle/config to change when calling `bundle install`
# with an environment variable.