[rubygems/rubygems] Give better conflict resolution advice

This alternative really uses only the Gemfile, and can never end up
being absurd, because it will never be suggested when there's no
lockfile, and it suggests deleting the lockfile.

rubygems/rubygems@5d154dd50e

lib/bundler/resolver.rb

@@ -395,7 +395,7 @@ def version_conflict_message(e)
             o << %(\n  Current #{name} version:\n    #{SharedHelpers.pretty_dependency(@metadata_requirements.find {|req| req.name == name })}\n\n)
           elsif conflict.locked_requirement
             o << "\n"
-            o << %(Running `bundle update` will rebuild your snapshot from scratch, using only\n)
+            o << %(Deleting your #{name_for_locking_dependency_source} file and running `bundle install` will rebuild your snapshot from scratch, using only\n)
             o << %(the gems in your Gemfile, which may resolve the conflict.\n)
           elsif !conflict.existing
             o << "\n"

spec/bundler/install/gems/flex_spec.rb

@@ -156,7 +156,7 @@
     end
   end
 
-  describe "when Gemfile conflicts with lockfile" do
+  describe "when running bundle install and Gemfile conflicts with lockfile" do
     before(:each) do
       build_repo2
       install_gemfile <<-G
@@ -190,7 +190,7 @@
       expect(err).to match(/could not find gem 'rack-obama/i)
     end
 
-    it "suggests bundle update when the Gemfile requires different versions than the lock" do
+    it "suggests deleting the Gemfile.lock file when the Gemfile requires different versions than the lock" do
       bundle "config set force_ruby_platform true"
 
       nice_error = <<-E.strip.gsub(/^ {8}/, "")
@@ -205,7 +205,7 @@
             rack_middleware was resolved to 1.0, which depends on
               rack (= 0.9.1)
 
-        Running `bundle update` will rebuild your snapshot from scratch, using only
+        Deleting your Gemfile.lock file and running `bundle install` will rebuild your snapshot from scratch, using only
         the gems in your Gemfile, which may resolve the conflict.
       E
 
@@ -214,6 +214,67 @@
     end
   end
 
+  describe "when running bundle update and Gemfile conflicts with lockfile" do
+    before(:each) do
+      build_repo4 do
+        build_gem "jekyll-feed", "0.16.0"
+        build_gem "jekyll-feed", "0.15.1"
+
+        build_gem "github-pages", "226" do |s|
+          s.add_dependency "jekyll-feed", "0.15.1"
+        end
+      end
+
+      install_gemfile <<-G
+        source "#{file_uri_for(gem_repo4)}"
+        gem "jekyll-feed", "~> 0.12"
+      G
+
+      puts lockfile
+      lockfile <<-L
+        GEM
+          remote: #{file_uri_for(gem_repo4)}/
+          specs:
+            jekyll-feed (0.16.0)
+
+        PLATFORMS
+          #{lockfile_platforms}
+
+        DEPENDENCIES
+          jekyll-feed
+
+        BUNDLED WITH
+           #{Bundler::VERSION}
+      L
+
+      gemfile <<-G
+        source "#{file_uri_for(gem_repo4)}"
+        gem "github-pages", "~> 226"
+        gem "jekyll-feed", "~> 0.12"
+      G
+    end
+
+    it "suggests deleting the Gemfile.lock file when the Gemfile requires different versions than the lock" do
+      nice_error = <<-E.strip.gsub(/^ {8}/, "")
+        Bundler could not find compatible versions for gem "jekyll-feed":
+          In snapshot (Gemfile.lock):
+            jekyll-feed (>= 0.16.0)
+
+          In Gemfile:
+            jekyll-feed (~> 0.12)
+
+            github-pages (~> 226) was resolved to 226, which depends on
+              jekyll-feed (= 0.15.1)
+
+        Deleting your Gemfile.lock file and running `bundle install` will rebuild your snapshot from scratch, using only
+        the gems in your Gemfile, which may resolve the conflict.
+      E
+
+      bundle :update, :raise_on_error => false
+      expect(err).to end_with(nice_error)
+    end
+  end
+
   describe "subtler cases" do
     before :each do
       install_gemfile <<-G