From d979b8bbb00281e62c0c089c65d4f9de98537652 Mon Sep 17 00:00:00 2001
From: "Samuel E. Giddins" <segiddins@segiddins.me>
Date: Sat, 2 May 2015 01:42:35 -0700
Subject: Warn when upgrading lockfile major version & error when major version
 is too low

---
 lib/bundler/definition.rb      |  8 ++++++++
 lib/bundler/lockfile_parser.rb | 17 ++++++++++++-----
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/lib/bundler/definition.rb b/lib/bundler/definition.rb
index d8aab4fbb4..edaf54a6bf 100644
--- a/lib/bundler/definition.rb
+++ b/lib/bundler/definition.rb
@@ -249,6 +249,14 @@ module Bundler
         return
       end
 
+      locked_major = @locked_bundler_version.segments.first
+      current_major = Gem::Version.create(Bundler::VERSION).segments.first
+
+      if locked_major < current_major
+        Bundler.ui.warn "Warning: the lockfile is being updated to Bundler #{Bundler::VERSION.split('.').first}, " \
+                        "after which you will be unable to return to Bundler #{@locked_bundler_version.segments.first}."
+      end
+
       File.open(file, 'wb'){|f| f.puts(contents) }
     rescue Errno::EACCES
       raise Bundler::InstallError,
diff --git a/lib/bundler/lockfile_parser.rb b/lib/bundler/lockfile_parser.rb
index dcd966b9d8..1c241b7a5a 100644
--- a/lib/bundler/lockfile_parser.rb
+++ b/lib/bundler/lockfile_parser.rb
@@ -60,11 +60,18 @@ module Bundler
     def warn_for_outdated_bundler_version
       return unless bundler_version
       prerelease_text = bundler_version.prerelease? ? " --pre" : ""
-      if Gem::Version.new(Bundler::VERSION) < Gem::Version.new(bundler_version)
-        Bundler.ui.warn "Warning: the running version of Bundler is older " \
-             "than the version that created the lockfile. We suggest you " \
-             "upgrade to the latest version of Bundler by running `gem " \
-             "install bundler#{prerelease_text}`.\n"
+      locked_version = Gem::Version.create(bundler_version)
+      current_version = Gem::Version.create(Bundler::VERSION)
+      case current_version.segments.first <=> locked_version.segments.first
+      when -1
+        raise LockfileError, "You must use Bundler #{locked_version.segments.first} or greater with this lockfile."
+      when 0
+        if current_version < locked_version
+          Bundler.ui.warn "Warning: the running version of Bundler is older " \
+               "than the version that created the lockfile. We suggest you " \
+               "upgrade to the latest version of Bundler by running `gem " \
+               "install bundler#{prerelease_text}`.\n"
+        end
       end
     end
 
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