Change default to Bundler v2 when unsupported feature flag is on and …

…Gemfile.lock exists (#10617)

* change the default fallback (when Gemlock file exists but version is not defined) version to Bundler v2 when v1 unsupported feature flag is enabled.

bundler/lib/dependabot/bundler/helpers.rb

@@ -1,35 +1,47 @@
-# typed: true
+# typed: strong
 # frozen_string_literal: true
 
 module Dependabot
   module Bundler
     module Helpers
+      extend T::Sig
+      extend T::Helpers
+
       V1 = "1"
       V2 = "2"
       # If we are updating a project with no Gemfile.lock, we default to the
       # newest version we support
       DEFAULT = V2
-      # If we are updating a project with a Gemfile.lock that does not specify
-      # the version it was bundled with, we failover to V1 on the assumption
-      # it was created with an old version that didn't add this information
-      FAILOVER = V1
-
       BUNDLER_MAJOR_VERSION_REGEX = /BUNDLED WITH\s+(?<version>\d+)\./m
 
+      sig { params(lockfile: T.nilable(Dependabot::DependencyFile)).returns(String) }
       def self.bundler_version(lockfile)
         return DEFAULT unless lockfile
 
-        if (matches = lockfile.content.match(BUNDLER_MAJOR_VERSION_REGEX))
+        if (matches = lockfile.content&.match(BUNDLER_MAJOR_VERSION_REGEX))
           matches[:version].to_i >= 2 ? V2 : V1
+        elsif Dependabot::Experiments.enabled?(:bundler_v1_unsupported_error)
+          DEFAULT
         else
-          FAILOVER
+          failover_version
         end
       end
 
+      # If we are updating a project with a Gemfile.lock that does not specify
+      # the version it was bundled with, we failover to V1 on the assumption
+      # it was created with an old version that didn't add this information
+      sig { returns(String) }
+      def self.failover_version
+        return V2 if Dependabot::Experiments.enabled?(:bundler_v1_unsupported_error)
+
+        V1
+      end
+
+      sig { params(lockfile: T.nilable(Dependabot::DependencyFile)).returns(String) }
       def self.detected_bundler_version(lockfile)
         return "unknown" unless lockfile
 
-        if (matches = lockfile.content.match(BUNDLER_MAJOR_VERSION_REGEX))
+        if (matches = lockfile.content&.match(BUNDLER_MAJOR_VERSION_REGEX))
           matches[:version].to_i.to_s
         else
           "unspecified"