With bundler, an OpenStruct can't have a field called 'gem' (and it's really hard to troubleshoot -- redefine_method)

After many hours of head-scratching, I can offer this bug report:

OpenStruct.new(gem: 'foobar').gem

The above code works in "bare" ruby, but not with bundler.

Steps to reproduce:

$ mkdir ostruct-bug

$ cd ostruct-bug

$ echo "source 'https://rubygems.org'" > Gemfile

$ bundle install
The Gemfile specifies no dependencies
Resolving dependencies...
Bundle complete! 0 Gemfile dependencies, 1 gem now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.

$ ruby -v
ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-darwin20]

$ bundle -v
Bundler version 2.1.4

$ ruby -e "require 'ostruct' ; puts OpenStruct.new(gem: 'foobar').gem"
foobar

$ bundle exec ruby -e "require 'ostruct' ; puts OpenStruct.new(gem: 'foobar').gem"
Traceback (most recent call last):
	1: from -e:1:in `<main>'
/Users/ikatz/.rbenv/versions/2.7.2/lib/ruby/2.7.0/bundler/rubygems_integration.rb:316:in `block (2 levels) in replace_gem': wrong number of arguments (given 0, expected 1+) (ArgumentError)

$ bundle exec ruby -e "require 'ostruct' ; puts OpenStruct.new(germ: 'foobar').germ"
foobar

Whether or not this is a problem with bundler (vs ostruct), the error is very unhelpful -- gem seems to all appearances like a perfectly valid attribute name and jumping into bundler source is generally not my first thought when debugging my own project.

The relevant bundler code is here:
https://github.com/rubygems/bundler/blob/master/lib/bundler/rubygems_integration.rb#L316

It would seem that the correct behavior in ostruct would be to check whether a method name is available (possibly with self.respond_to?(new_attribute_name) at the time the attribute is being defined , so that it can produce a warning or exception immediately instead of cryptic behavior when the attribute is later accessed.

Reproduce the failure with OpenStruct.new(gem: 'foobar').gem under ruby and bundle exec, then inspect Bundler's lib/bundler/rubygems_integration.rb around line 316. Determine whether the behavior belongs in OpenStruct or Bundler, and add coverage showing that the chosen fix makes the attribute usable and avoids the cryptic error.