Merge branch 'dm-outdated-system-note' into 'master'

Add system note with link to diff comparison when MR discussion becomes outdated

Closes #30058

See merge request !11584

33 files changed, 635 insertions, 247 deletions

diff --git a/app/assets/stylesheets/framework/timeline.scss b/app/assets/stylesheets/framework/timeline.scss
index aa0c512a277..515e6f0b2b1 100644
--- a/app/assets/stylesheets/framework/timeline.scss
+++ b/app/assets/stylesheets/framework/timeline.scss
@@ -5,7 +5,7 @@
 
   .note-text {
     p:last-child {
-      margin-bottom: 0;
+      margin-bottom: 0 !important;
     }
   }
 
diff --git a/app/assets/stylesheets/pages/note_form.scss b/app/assets/stylesheets/pages/note_form.scss
index 9db26f99a75..49e453c7dbc 100644
--- a/app/assets/stylesheets/pages/note_form.scss
+++ b/app/assets/stylesheets/pages/note_form.scss
@@ -164,10 +164,6 @@
 
 .discussion-body,
 .diff-file {
-  .notes .note {
-    padding: 10px 15px;
-  }
-
   .discussion-reply-holder {
     background-color: $white-light;
     padding: 10px 16px;
diff --git a/app/assets/stylesheets/pages/notes.scss b/app/assets/stylesheets/pages/notes.scss
index 4b15fc2bd82..5550c0d5b47 100644
--- a/app/assets/stylesheets/pages/notes.scss
+++ b/app/assets/stylesheets/pages/notes.scss
@@ -80,10 +80,6 @@ ul.notes {
       &.timeline-entry {
         padding: 14px 10px;
       }
-
-      .system-note {
-        padding: 0;
-      }
     }
 
     &.is-editing {
@@ -380,6 +376,10 @@ ul.notes {
   padding-bottom: 5px;
 }
 
+.system-note .note-header-info {
+  padding-bottom: 0;
+}
+
 .note-headline-light {
   display: inline;
 
@@ -582,6 +582,17 @@ ul.notes {
   }
 }
 
+.discussion-body,
+.diff-file {
+  .notes .note {
+    padding: 10px 15px;
+
+    &.system-note {
+      padding: 0;
+    }
+  }
+}
+
 .diff-file {
   .is-over {
     .add-diff-note {
diff --git a/app/helpers/system_note_helper.rb b/app/helpers/system_note_helper.rb
index d889d141101..209bd56b78a 100644
--- a/app/helpers/system_note_helper.rb
+++ b/app/helpers/system_note_helper.rb
@@ -17,7 +17,8 @@ module SystemNoteHelper
     'visible' => 'icon_eye',
     'milestone' => 'icon_clock_o',
     'discussion' => 'icon_comment_o',
-    'moved' => 'icon_arrow_circle_o_right'
+    'moved' => 'icon_arrow_circle_o_right',
+    'outdated' => 'icon_edit'
   }.freeze
 
   def icon_for_system_note(note)
diff --git a/app/models/concerns/discussion_on_diff.rb b/app/models/concerns/discussion_on_diff.rb
index a7bdf5587b2..eee1a36ac6b 100644
--- a/app/models/concerns/discussion_on_diff.rb
+++ b/app/models/concerns/discussion_on_diff.rb
@@ -47,4 +47,12 @@ module DiscussionOnDiff
 
     prev_lines
   end
+
+  def line_code_in_diffs(diff_refs)
+    if active?(diff_refs)
+      line_code
+    elsif diff_refs && created_at_diff?(diff_refs)
+      original_line_code
+    end
+  end
 end
diff --git a/app/models/diff_discussion.rb b/app/models/diff_discussion.rb
index 14ddd2fcc88..800574d8be0 100644
--- a/app/models/diff_discussion.rb
+++ b/app/models/diff_discussion.rb
@@ -19,21 +19,9 @@ class DiffDiscussion < Discussion
 
   def merge_request_version_params
     return unless for_merge_request?
+    return {} if active?
 
-    if active?
-      {}
-    else
-      diff_refs = position.diff_refs
-
-      if diff = noteable.merge_request_diff_for(diff_refs)
-        { diff_id: diff.id }
-      elsif diff = noteable.merge_request_diff_for(diff_refs.head_sha)
-        {
-          diff_id: diff.id,
-          start_sha: diff_refs.start_sha
-        }
-      end
-    end
+    noteable.version_params_for(position.diff_refs)
   end
 
   def reply_attributes
diff --git a/app/models/diff_note.rb b/app/models/diff_note.rb
index 76c59199afd..1764004078e 100644
--- a/app/models/diff_note.rb
+++ b/app/models/diff_note.rb
@@ -8,6 +8,7 @@ class DiffNote < Note
 
   serialize :original_position, Gitlab::Diff::Position
   serialize :position, Gitlab::Diff::Position
+  serialize :change_position, Gitlab::Diff::Position
 
   validates :original_position, presence: true
   validates :position, presence: true
@@ -25,7 +26,7 @@ class DiffNote < Note
     DiffDiscussion
   end
 
-  %i(original_position position).each do |meth|
+  %i(original_position position change_position).each do |meth|
     define_method "#{meth}=" do |new_position|
       if new_position.is_a?(String)
         new_position = JSON.parse(new_position) rescue nil
@@ -36,6 +37,8 @@ class DiffNote < Note
         new_position = Gitlab::Diff::Position.new(new_position)
       end
 
+      return if new_position == read_attribute(meth)
+
       super(new_position)
     end
   end
@@ -45,7 +48,7 @@ class DiffNote < Note
   end
 
   def diff_line
-    @diff_line ||= diff_file.line_for_position(self.original_position) if diff_file
+    @diff_line ||= diff_file&.line_for_position(self.original_position)
   end
 
   def for_line?(line)
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 9be00880438..2eec013fa9d 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -416,13 +416,24 @@ class MergeRequest < ActiveRecord::Base
     @merge_request_diffs_by_diff_refs_or_sha[diff_refs_or_sha]
   end
 
+  def version_params_for(diff_refs)
+    if diff = merge_request_diff_for(diff_refs)
+      { diff_id: diff.id }
+    elsif diff = merge_request_diff_for(diff_refs.head_sha)
+      {
+        diff_id: diff.id,
+        start_sha: diff_refs.start_sha
+      }
+    end
+  end
+
   def reload_diff_if_branch_changed
     if source_branch_changed? || target_branch_changed?
       reload_diff
     end
   end
 
-  def reload_diff
+  def reload_diff(current_user = nil)
     return unless open?
 
     old_diff_refs = self.diff_refs
@@ -432,7 +443,8 @@ class MergeRequest < ActiveRecord::Base
 
     update_diff_notes_positions(
       old_diff_refs: old_diff_refs,
-      new_diff_refs: new_diff_refs
+      new_diff_refs: new_diff_refs,
+      current_user: current_user
     )
   end
 
@@ -861,7 +873,7 @@ class MergeRequest < ActiveRecord::Base
     diff_sha_refs && diff_sha_refs.complete?
   end
 
-  def update_diff_notes_positions(old_diff_refs:, new_diff_refs:)
+  def update_diff_notes_positions(old_diff_refs:, new_diff_refs:, current_user: nil)
     return unless has_complete_diff_refs?
     return if new_diff_refs == old_diff_refs
 
@@ -875,7 +887,7 @@ class MergeRequest < ActiveRecord::Base
 
     service = Notes::DiffPositionUpdateService.new(
       self.project,
-      nil,
+      current_user,
       old_diff_refs: old_diff_refs,
       new_diff_refs: new_diff_refs,
       paths: paths
diff --git a/app/models/merge_request_diff.rb b/app/models/merge_request_diff.rb
index f0a3c30ea74..6e3917a10a3 100644
--- a/app/models/merge_request_diff.rb
+++ b/app/models/merge_request_diff.rb
@@ -175,12 +175,11 @@ class MergeRequestDiff < ActiveRecord::Base
     self == merge_request.merge_request_diff
   end
 
-  def compare_with(sha, straight: true)
+  def compare_with(sha)
     # When compare merge request versions we want diff A..B instead of A...B
     # so we handle cases when user does squash and rebase of the commits between versions.
     # For this reason we set straight to true by default.
-    CompareService.new(project, head_commit_sha)
-      .execute(project, sha, straight: straight)
+    CompareService.new(project, head_commit_sha).execute(project, sha, straight: true)
   end
 
   def commits_count
diff --git a/app/models/note.rb b/app/models/note.rb
index 46d0a4f159f..60257aac93b 100644
--- a/app/models/note.rb
+++ b/app/models/note.rb
@@ -124,13 +124,12 @@ class Note < ActiveRecord::Base
       groups = {}
 
       diff_notes.fresh.discussions.each do |discussion|
-        if discussion.active?(diff_refs)
-          discussions = groups[discussion.line_code] ||= []
-        elsif diff_refs && discussion.created_at_diff?(diff_refs)
-          discussions = groups[discussion.original_line_code] ||= []
-        end
+        line_code = discussion.line_code_in_diffs(diff_refs)
 
-        discussions << discussion if discussions
+        if line_code
+          discussions = groups[line_code] ||= []
+          discussions << discussion
+        end
       end
 
       groups
diff --git a/app/models/system_note_metadata.rb b/app/models/system_note_metadata.rb
index b44f4fe000c..414c95f7705 100644
--- a/app/models/system_note_metadata.rb
+++ b/app/models/system_note_metadata.rb
@@ -2,6 +2,7 @@ class SystemNoteMetadata < ActiveRecord::Base
   ICON_TYPES = %w[
     commit description merge confidential visible label assignee cross_reference
     title time_tracking branch milestone discussion task moved opened closed merged
+    outdated
   ].freeze
 
   validates :note, presence: true
diff --git a/app/services/merge_requests/refresh_service.rb b/app/services/merge_requests/refresh_service.rb
index 1131d6f4913..81d217929d5 100644
--- a/app/services/merge_requests/refresh_service.rb
+++ b/app/services/merge_requests/refresh_service.rb
@@ -66,12 +66,12 @@ module MergeRequests
 
       filter_merge_requests(merge_requests).each do |merge_request|
         if merge_request.source_branch == @branch_name || force_push?
-          merge_request.reload_diff
+          merge_request.reload_diff(current_user)
         else
           mr_commit_ids = merge_request.commits_sha
           push_commit_ids = @commits.map(&:id)
           matches = mr_commit_ids & push_commit_ids
-          merge_request.reload_diff if matches.any?
+          merge_request.reload_diff(current_user) if matches.any?
         end
 
         merge_request.mark_as_unchecked
diff --git a/app/services/merge_requests/reopen_service.rb b/app/services/merge_requests/reopen_service.rb
index fadcce5d9b6..54b19e6d651 100644
--- a/app/services/merge_requests/reopen_service.rb
+++ b/app/services/merge_requests/reopen_service.rb
@@ -8,7 +8,7 @@ module MergeRequests
         create_note(merge_request)
         notification_service.reopen_mr(merge_request, current_user)
         execute_hooks(merge_request, 'reopen')
-        merge_request.reload_diff
+        merge_request.reload_diff(current_user)
         merge_request.mark_as_unchecked
       end
 
diff --git a/app/services/notes/diff_position_update_service.rb b/app/services/notes/diff_position_update_service.rb
index 0cb731f5bc3..eff7b287269 100644
--- a/app/services/notes/diff_position_update_service.rb
+++ b/app/services/notes/diff_position_update_service.rb
@@ -1,26 +1,29 @@
 module Notes
   class DiffPositionUpdateService < BaseService
     def execute(note)
-      new_position = tracer.trace(note.position)
+      results = tracer.trace(note.position)
+      return unless results
 
-      # Don't update the position if the type doesn't match, since that means
-      # the diff line commented on was changed, and the comment is now outdated
-      old_position = note.position
-      if new_position &&
-          new_position != old_position &&
-          new_position.type == old_position.type
+      position = results[:position]
+      outdated = results[:outdated]
 
-        note.position = new_position
-      end
+      if outdated
+        note.change_position = position
 
-      note
+        if note.persisted? && current_user
+          SystemNoteService.diff_discussion_outdated(note.to_discussion, project, current_user, position)
+        end
+      else
+        note.position = position
+        note.change_position = nil
+      end
     end
 
     private
 
     def tracer
       @tracer ||= Gitlab::Diff::PositionTracer.new(
-        repository: project.repository,
+        project: project,
         old_diff_refs: params[:old_diff_refs],
         new_diff_refs: params[:new_diff_refs],
         paths: params[:paths]
diff --git a/app/services/system_note_service.rb b/app/services/system_note_service.rb
index 93bf1fb1615..0837c07e6aa 100644
--- a/app/services/system_note_service.rb
+++ b/app/services/system_note_service.rb
@@ -258,7 +258,7 @@ module SystemNoteService
     create_note(NoteSummary.new(noteable, project, author, body, action: 'title'))
   end
 
-  def self.resolve_all_discussions(merge_request, project, author)
+  def resolve_all_discussions(merge_request, project, author)
     body = "resolved all discussions"
 
     create_note(NoteSummary.new(merge_request, project, author, body, action: 'discussion'))
@@ -274,6 +274,28 @@ module SystemNoteService
     note
   end
 
+  def diff_discussion_outdated(discussion, project, author, change_position)
+    merge_request = discussion.noteable
+    diff_refs = change_position.diff_refs
+    version_index = merge_request.merge_request_diffs.viewable.count
+
+    body = "changed this line in"
+    if version_params = merge_request.version_params_for(diff_refs)
+      line_code = change_position.line_code(project.repository)
+      url = url_helpers.diffs_namespace_project_merge_request_url(project.namespace, project, merge_request, version_params.merge(anchor: line_code))
+
+      body << " [version #{version_index} of the diff](#{url})"
+    else
+      body << " version #{version_index} of the diff"
+    end
+
+    note_attributes = discussion.reply_attributes.merge(project: project, author: author, note: body)
+    note = Note.create(note_attributes.merge(system: true))
+    note.system_note_metadata = SystemNoteMetadata.new(action: 'outdated')
+
+    note
+  end
+
   # Called when the title of a Noteable is changed
   #
   # noteable  - Noteable object that responds to `title`
diff --git a/app/views/discussions/_discussion.html.haml b/app/views/discussions/_discussion.html.haml
index 74992e439f3..578e751ab47 100644
--- a/app/views/discussions/_discussion.html.haml
+++ b/app/views/discussions/_discussion.html.haml
@@ -32,10 +32,9 @@
             - elsif discussion.diff_discussion?
               on
               = conditional_link_to url.present?, url do
-                - if discussion.active?
-                  the diff
-                - else
-                  an outdated diff
+                - unless discussion.active?
+                  an old version of
+                the diff
 
             = time_ago_with_tooltip(discussion.created_at, placement: "bottom", html_class: "note-created-ago")
           = render "discussions/headline", discussion: discussion
diff --git a/app/views/projects/merge_requests/show/_versions.html.haml b/app/views/projects/merge_requests/show/_versions.html.haml
index 37117bc64a3..0999b95c9c9 100644
--- a/app/views/projects/merge_requests/show/_versions.html.haml
+++ b/app/views/projects/merge_requests/show/_versions.html.haml
@@ -91,7 +91,7 @@
           comparing two versions
         - else
           viewing an old version
-        of this merge request.
+        of the diff.
 
         .pull-right
           = link_to 'Show latest version', diffs_namespace_project_merge_request_path(@project.namespace, @project, @merge_request), class: 'btn btn-sm'
diff --git a/changelogs/unreleased/dm-outdated-system-note.yml b/changelogs/unreleased/dm-outdated-system-note.yml
new file mode 100644
index 00000000000..a1038a1051b
--- /dev/null
+++ b/changelogs/unreleased/dm-outdated-system-note.yml
@@ -0,0 +1,4 @@
+---
+title: Add system note with link to diff comparison when MR discussion becomes outdated
+merge_request:
+author:
diff --git a/db/migrate/20170521184006_add_change_position_to_notes.rb b/db/migrate/20170521184006_add_change_position_to_notes.rb
new file mode 100644
index 00000000000..219ed1ade4c
--- /dev/null
+++ b/db/migrate/20170521184006_add_change_position_to_notes.rb
@@ -0,0 +1,13 @@
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddChangePositionToNotes < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  def change
+    add_column :notes, :change_position, :text
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index d14126401c9..84e25427d7f 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170518231126) do
+ActiveRecord::Schema.define(version: 20170521184006) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -794,6 +794,7 @@ ActiveRecord::Schema.define(version: 20170518231126) do
     t.string "discussion_id"
     t.text "note_html"
     t.integer "cached_markdown_version"
+    t.text "change_position"
   end
 
   add_index "notes", ["author_id"], name: "index_notes_on_author_id", using: :btree
diff --git a/lib/gitlab/diff/file_collection/base.rb b/lib/gitlab/diff/file_collection/base.rb
index 2b9fc65b985..7c32adc6ce7 100644
--- a/lib/gitlab/diff/file_collection/base.rb
+++ b/lib/gitlab/diff/file_collection/base.rb
@@ -24,6 +24,14 @@ module Gitlab
           @diff_files ||= @diffs.decorate! { |diff| decorate_diff!(diff) }
         end
 
+        def diff_file_with_old_path(old_path)
+          diff_files.find { |diff_file| diff_file.old_path == old_path }
+        end
+
+        def diff_file_with_new_path(new_path)
+          diff_files.find { |diff_file| diff_file.new_path == new_path }
+        end
+
         private
 
         def decorate_diff!(diff)
diff --git a/lib/gitlab/diff/position.rb b/lib/gitlab/diff/position.rb
index fc728123c97..4d96778a2b2 100644
--- a/lib/gitlab/diff/position.rb
+++ b/lib/gitlab/diff/position.rb
@@ -12,20 +12,26 @@ module Gitlab
       attr_reader :head_sha
 
       def initialize(attrs = {})
+        if diff_file = attrs[:diff_file]
+          attrs[:diff_refs] = diff_file.diff_refs
+          attrs[:old_path] = diff_file.old_path
+          attrs[:new_path] = diff_file.new_path
+        end
+
+        if diff_refs = attrs[:diff_refs]
+          attrs[:base_sha]  = diff_refs.base_sha
+          attrs[:start_sha] = diff_refs.start_sha
+          attrs[:head_sha]  = diff_refs.head_sha
+        end
+
         @old_path = attrs[:old_path]
         @new_path = attrs[:new_path]
+        @base_sha  = attrs[:base_sha]
+        @start_sha = attrs[:start_sha]
+        @head_sha  = attrs[:head_sha]
+
         @old_line = attrs[:old_line]
         @new_line = attrs[:new_line]
-
-        if attrs[:diff_refs]
-          @base_sha  = attrs[:diff_refs].base_sha
-          @start_sha = attrs[:diff_refs].start_sha
-          @head_sha  = attrs[:diff_refs].head_sha
-        else
-          @base_sha  = attrs[:base_sha]
-          @start_sha = attrs[:start_sha]
-          @head_sha  = attrs[:head_sha]
-        end
       end
 
       # `Gitlab::Diff::Position` objects are stored as serialized attributes in
@@ -129,11 +135,11 @@ module Gitlab
       end
 
       def diff_line(repository)
-        @diff_line ||= diff_file(repository).line_for_position(self)
+        @diff_line ||= diff_file(repository)&.line_for_position(self)
       end
 
       def line_code(repository)
-        @line_code ||= diff_file(repository).line_code_for_position(self)
+        @line_code ||= diff_file(repository)&.line_code_for_position(self)
       end
 
       private
diff --git a/lib/gitlab/diff/position_tracer.rb b/lib/gitlab/diff/position_tracer.rb
index e89ff238ec7..dcabb5f7fe5 100644
--- a/lib/gitlab/diff/position_tracer.rb
+++ b/lib/gitlab/diff/position_tracer.rb
@@ -3,21 +3,21 @@
 module Gitlab
   module Diff
     class PositionTracer
-      attr_accessor :repository
+      attr_accessor :project
       attr_accessor :old_diff_refs
       attr_accessor :new_diff_refs
       attr_accessor :paths
 
-      def initialize(repository:, old_diff_refs:, new_diff_refs:, paths: nil)
-        @repository = repository
+      def initialize(project:, old_diff_refs:, new_diff_refs:, paths: nil)
+        @project = project
         @old_diff_refs = old_diff_refs
         @new_diff_refs = new_diff_refs
         @paths = paths
       end
 
-      def trace(old_position)
+      def trace(ab_position)
         return unless old_diff_refs&.complete? && new_diff_refs&.complete?
-        return unless old_position.diff_refs == old_diff_refs
+        return unless ab_position.diff_refs == old_diff_refs
 
         # Suppose we have an MR with source branch `feature` and target branch `master`.
         # When the MR was created, the head of `master` was commit A, and the
@@ -44,14 +44,16 @@ module Gitlab
         #
         # For diff notes for diff A->B, the position looks like this:
         # Position
-        #   base_sha - ID of commit A
+        #   start_sha - ID of commit A
         #   head_sha - ID of commit B
+        #   base_sha - ID of base commit of A and B
         #   old_path - path as of A (nil if file was newly created)
         #   new_path - path as of B (nil if file was deleted)
         #   old_line - line number as of A (nil if file was newly created)
         #   new_line - line number as of B (nil if file was deleted)
         #
-        # We can easily update `base_sha` and `head_sha` to hold the IDs of commits C and D,
+        # We can easily update `start_sha` and `head_sha` to hold the IDs of
+        # commits C and D, and can trivially determine `base_sha` based on those,
         # but need to find the paths and line numbers as of C and D.
         #
         # If the file was unchanged or newly created in A->B, the path as of D can be found
@@ -68,107 +70,161 @@ module Gitlab
         # by generating diff A->C ("base to base"), finding the diff file with
         # `diff_file.old_path == position.old_path`, and taking `diff_file.new_path`.
         # The path as of D can be found by taking diff C->D, finding the diff file
-        # with that same `old_path` and taking `diff_file.new_path`.
+        # with `old_path` set to that `diff_file.new_path` and taking `diff_file.new_path`.
         # The line number as of C can be found by using the LineMapper on diff A->C
         # and providing the line number as of A.
         # The line number as of D can be found by using the LineMapper on diff C->D
         # and providing the line number as of C.
 
-        results = nil
-        results ||= trace_added_line(old_position)   if old_position.added?   || old_position.unchanged?
-        results ||= trace_removed_line(old_position) if old_position.removed? || old_position.unchanged?
-
-        return unless results
-
-        file_diff, old_line, new_line = results
-
-        new_position = Position.new(
-          old_path: file_diff.old_path,
-          new_path: file_diff.new_path,
-          head_sha: new_diff_refs.head_sha,
-          start_sha: new_diff_refs.start_sha,
-          base_sha: new_diff_refs.base_sha,
-          old_line: old_line,
-          new_line: new_line
-        )
-
-        # If a position is found, but is not actually contained in the diff, for example
-        # because it was an unchanged line in the context of a change that was undone,
-        # we cannot return this as a successful trace.
-        return unless new_position.diff_line(repository)
-
-        new_position
+        if ab_position.added?
+          trace_added_line(ab_position)
+        elsif ab_position.removed?
+          trace_removed_line(ab_position)
+        else # unchanged
+          trace_unchanged_line(ab_position)
+        end
       end
 
       private
 
-      def trace_added_line(old_position)
-        file_path = old_position.new_path
-
-        return unless diff_head_to_head
-
-        file_head_to_head = diff_head_to_head.find { |diff_file| diff_file.old_path == file_path }
-
-        file_path = file_head_to_head.new_path if file_head_to_head
-
-        new_line = LineMapper.new(file_head_to_head).old_to_new(old_position.new_line)
-
-        return unless new_line
-
-        file_diff = new_diffs.find { |diff_file| diff_file.new_path == file_path }
-        return unless file_diff
-
-        old_line = LineMapper.new(file_diff).new_to_old(new_line)
-
-        [file_diff, old_line, new_line]
+      def trace_added_line(ab_position)
+        b_path = ab_position.new_path
+        b_line = ab_position.new_line
+
+        bd_diff = bd_diffs.diff_file_with_old_path(b_path)
+
+        d_path = bd_diff&.new_path || b_path
+        d_line = LineMapper.new(bd_diff).old_to_new(b_line)
+
+        if d_line
+          cd_diff = cd_diffs.diff_file_with_new_path(d_path)
+
+          c_path = cd_diff&.old_path || d_path
+          c_line = LineMapper.new(cd_diff).new_to_old(d_line)
+
+          if c_line
+            # If the line is still in D but also in C, it has turned from an
+            # added line into an unchanged one.
+            new_position = position(cd_diff, c_line, d_line)
+            if valid_position?(new_position)
+              # If the line is still in the MR, we don't treat this as outdated.
+              { position: new_position, outdated: false }
+            else
+              # If the line is no longer in the MR, we unfortunately cannot show
+              # the current state on the CD diff, so we treat it as outdated.
+              ac_diff = ac_diffs.diff_file_with_new_path(c_path)
+
+              { position: position(ac_diff, nil, c_line), outdated: true }
+            end
+          else
+            # If the line is still in D and not in C, it is still added.
+            { position: position(cd_diff, nil, d_line), outdated: false }
+          end
+        else
+          # If the line is no longer in D, it has been removed from the MR.
+          { position: position(bd_diff, b_line, nil), outdated: true }
+        end
       end
 
-      def trace_removed_line(old_position)
-        file_path = old_position.old_path
+      def trace_removed_line(ab_position)
+        a_path = ab_position.old_path
+        a_line = ab_position.old_line
 
-        return unless diff_base_to_base
+        ac_diff = ac_diffs.diff_file_with_old_path(a_path)
 
-        file_base_to_base = diff_base_to_base.find { |diff_file| diff_file.old_path == file_path }
+        c_path = ac_diff&.new_path || a_path
+        c_line = LineMapper.new(ac_diff).old_to_new(a_line)
 
-        file_path = file_base_to_base.old_path if file_base_to_base
+        if c_line
+          cd_diff = cd_diffs.diff_file_with_old_path(c_path)
 
-        old_line = LineMapper.new(file_base_to_base).old_to_new(old_position.old_line)
+          d_path = cd_diff&.new_path || c_path
+          d_line = LineMapper.new(cd_diff).old_to_new(c_line)
 
-        return unless old_line
+          if d_line
+            # If the line is still in C but also in D, it has turned from a
+            # removed line into an unchanged one.
+            bd_diff = bd_diffs.diff_file_with_new_path(d_path)
 
-        file_diff = new_diffs.find { |diff_file| diff_file.old_path == file_path }
-        return unless file_diff
-
-        new_line = LineMapper.new(file_diff).old_to_new(old_line)
+            { position: position(bd_diff, nil, d_line), outdated: true }
+          else
+            # If the line is still in C and not in D, it is still removed.
+            { position: position(cd_diff, c_line, nil), outdated: false }
+          end
+        else
+          # If the line is no longer in C, it has been removed outside of the MR.
+          { position: position(ac_diff, a_line, nil), outdated: true }
+        end
+      end
 
-        [file_diff, old_line, new_line]
+      def trace_unchanged_line(ab_position)
+        a_path = ab_position.old_path
+        a_line = ab_position.old_line
+        b_path = ab_position.new_path
+        b_line = ab_position.new_line
+
+        ac_diff = ac_diffs.diff_file_with_old_path(a_path)
+
+        c_path = ac_diff&.new_path || a_path
+        c_line = LineMapper.new(ac_diff).old_to_new(a_line)
+
+        bd_diff = bd_diffs.diff_file_with_old_path(b_path)
+
+        d_line = LineMapper.new(bd_diff).old_to_new(b_line)
+
+        cd_diff = cd_diffs.diff_file_with_old_path(c_path)
+
+        if c_line && d_line
+          # If the line is still in C and D, it is still unchanged.
+          new_position = position(cd_diff, c_line, d_line)
+          if valid_position?(new_position)
+            # If the line is still in the MR, we don't treat this as outdated.
+            { position: new_position, outdated: false }
+          else
+            # If the line is no longer in the MR, we unfortunately cannot show
+            # the current state on the CD diff or any change on the BD diff,
+            # so we treat it as outdated.
+            { position: nil, outdated: true }
+          end
+        elsif d_line # && !c_line
+          # If the line is still in D but no longer in C, it has turned from
+          # an unchanged line into an added one.
+          # We don't treat this as outdated since the line is still in the MR.
+          { position: position(cd_diff, nil, d_line), outdated: false }
+        else # !d_line && (c_line || !c_line)
+          # If the line is no longer in D, it has turned from an unchanged line
+          # into a removed one.
+          { position: position(bd_diff, b_line, nil), outdated: true }
+        end
       end
 
-      def diff_base_to_base
-        @diff_base_to_base ||= diff_files(old_diff_refs.base_sha || old_diff_refs.start_sha, new_diff_refs.base_sha || new_diff_refs.start_sha)
+      def ac_diffs
+        @ac_diffs ||= compare(
+          old_diff_refs.base_sha || old_diff_refs.start_sha,
+          new_diff_refs.base_sha || new_diff_refs.start_sha,
+          straight: true
+        )
       end
 
-      def diff_head_to_head
-        @diff_head_to_head ||= diff_files(old_diff_refs.head_sha, new_diff_refs.head_sha)
+      def bd_diffs
+        @bd_diffs ||= compare(old_diff_refs.head_sha, new_diff_refs.head_sha, straight: true)
       end
 
-      def new_diffs
-        @new_diffs ||= diff_files(new_diff_refs.start_sha, new_diff_refs.head_sha, use_base: true)
+      def cd_diffs
+        @cd_diffs ||= compare(new_diff_refs.start_sha, new_diff_refs.head_sha)
       end
 
-      def diff_files(start_sha, head_sha, use_base: false)
-        base_sha = self.repository.merge_base(start_sha, head_sha) || start_sha
+      def compare(start_sha, head_sha, straight: false)
+        compare = CompareService.new(project, head_sha).execute(project, start_sha, straight: straight)
+        compare.diffs(paths: paths)
+      end
 
-        diffs = self.repository.raw_repository.diff(
-          use_base ? base_sha : start_sha,
-          head_sha,
-          {},
-          *paths
-        )
+      def position(diff_file, old_line, new_line)
+        Position.new(diff_file: diff_file, old_line: old_line, new_line: new_line)
+      end
 
-        diffs.decorate! do |diff|
-          Gitlab::Diff::File.new(diff, repository: self.repository)
-        end
+      def valid_position?(position)
+        !!position.diff_line(project.repository)
       end
     end
   end
diff --git a/spec/features/merge_requests/discussion_spec.rb b/spec/features/merge_requests/discussion_spec.rb
index f59d0faa274..1a09cc54c2e 100644
--- a/spec/features/merge_requests/discussion_spec.rb
+++ b/spec/features/merge_requests/discussion_spec.rb
@@ -43,7 +43,7 @@ feature 'Merge Request Discussions', feature: true do
       it 'shows a link to the outdated diff' do
         within(".discussion[data-discussion-id='#{outdated_discussion.id}']") do
           path = diffs_namespace_project_merge_request_path(project.namespace, project, merge_request, diff_id: old_merge_request_diff.id, anchor: outdated_discussion.line_code)
-          expect(page).to have_link('an outdated diff', href: path)
+          expect(page).to have_link('an old version of the diff', href: path)
         end
       end
     end
diff --git a/spec/features/merge_requests/versions_spec.rb b/spec/features/merge_requests/versions_spec.rb
index aef8ef20938..aad522ee26e 100644
--- a/spec/features/merge_requests/versions_spec.rb
+++ b/spec/features/merge_requests/versions_spec.rb
@@ -124,6 +124,8 @@ feature 'Merge Request versions', js: true, feature: true do
         diff_refs: merge_request_diff3.compare_with(merge_request_diff1.head_commit_sha).diff_refs
       )
       outdated_diff_note = create(:diff_note_on_merge_request, project: project, noteable: merge_request, position: position)
+      outdated_diff_note.position = outdated_diff_note.original_position
+      outdated_diff_note.save!
 
       visit current_url
       wait_for_requests
diff --git a/spec/helpers/notes_helper_spec.rb b/spec/helpers/notes_helper_spec.rb
index 099146678ae..355a4845afb 100644
--- a/spec/helpers/notes_helper_spec.rb
+++ b/spec/helpers/notes_helper_spec.rb
@@ -92,7 +92,13 @@ describe NotesHelper do
             )
           end
 
-          let(:discussion) { create(:diff_note_on_merge_request, noteable: merge_request, project: project, position: position).to_discussion }
+          let(:diff_note) { create(:diff_note_on_merge_request, noteable: merge_request, project: project, position: position) }
+          let(:discussion) { diff_note.to_discussion }
+
+          before do
+            diff_note.position = diff_note.original_position
+            diff_note.save!
+          end
 
           it 'returns the diff version comparison path with the line code' do
             expect(helper.discussion_path(discussion)).to eq(diffs_namespace_project_merge_request_path(project.namespace, project, merge_request, diff_id: merge_request_diff3, start_sha: merge_request_diff1.head_commit_sha, anchor: discussion.line_code))
diff --git a/spec/lib/gitlab/diff/position_tracer_spec.rb b/spec/lib/gitlab/diff/position_tracer_spec.rb
index 4d202a76e1b..93d30b90937 100644
--- a/spec/lib/gitlab/diff/position_tracer_spec.rb
+++ b/spec/lib/gitlab/diff/position_tracer_spec.rb
@@ -61,9 +61,10 @@ describe Gitlab::Diff::PositionTracer, lib: true do
 
   let(:old_diff_refs) { raise NotImplementedError }
   let(:new_diff_refs) { raise NotImplementedError }
+  let(:change_diff_refs) { raise NotImplementedError }
   let(:old_position) { raise NotImplementedError }
 
-  let(:position_tracer) { described_class.new(repository: project.repository, old_diff_refs: old_diff_refs, new_diff_refs: new_diff_refs) }
+  let(:position_tracer) { described_class.new(project: project, old_diff_refs: old_diff_refs, new_diff_refs: new_diff_refs) }
   subject { position_tracer.trace(old_position) }
 
   def diff_refs(base_commit, head_commit)
@@ -77,16 +78,40 @@ describe Gitlab::Diff::PositionTracer, lib: true do
     Gitlab::Diff::Position.new(attrs)
   end
 
-  def expect_new_position(attrs, new_position = subject)
-    if attrs.nil?
-      expect(new_position).to be_nil
-    else
-      expect(new_position).not_to be_nil
+  def expect_new_position(attrs, result = subject)
+    aggregate_failures("expect new position #{attrs.inspect}") do
+      if attrs.nil?
+        expect(result[:outdated]).to be_truthy
+      else
+        expect(result[:outdated]).to be_falsey
 
-      expect(new_position.diff_refs).to eq(new_diff_refs)
+        new_position = result[:position]
+        expect(new_position).not_to be_nil
 
-      attrs.each do |attr, value|
-        expect(new_position.send(attr)).to eq(value)
+        expect(new_position.diff_refs).to eq(new_diff_refs)
+
+        attrs.each do |attr, value|
+          expect(new_position.send(attr)).to eq(value)
+        end
+      end
+    end
+  end
+
+  def expect_change_position(attrs, result = subject)
+    aggregate_failures("expect change position #{attrs.inspect}") do
+      expect(result[:outdated]).to be_truthy
+
+      change_position = result[:position]
+      if attrs.nil? || attrs.empty?
+        expect(change_position).to be_nil
+      else
+        expect(change_position).not_to be_nil
+
+        expect(change_position.diff_refs).to eq(change_diff_refs)
+
+        attrs.each do |attr, value|
+          expect(change_position.send(attr)).to eq(value)
+        end
       end
     end
   end
@@ -395,6 +420,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
               context "when that line was changed between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(initial_commit, create_file_commit) }
                 let(:new_diff_refs) { diff_refs(initial_commit, update_line_commit) }
+                let(:change_diff_refs) { diff_refs(create_file_commit, update_line_commit) }
                 let(:old_position) { position(new_path: file_name, new_line: 2) }
 
                 # old diff:
@@ -407,14 +433,20 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 #   2 + BB
                 #   3 + C
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: 2,
+                    new_line: nil
+                  )
                 end
               end
 
               context "when that line was deleted between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(initial_commit, update_line_commit) }
                 let(:new_diff_refs) { diff_refs(initial_commit, delete_line_commit) }
+                let(:change_diff_refs) { diff_refs(update_line_commit, delete_line_commit) }
                 let(:old_position) { position(new_path: file_name, new_line: 3) }
 
                 # old diff:
@@ -426,8 +458,13 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 #   1 + A
                 #   2 + BB
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: 3,
+                    new_line: nil
+                  )
                 end
               end
             end
@@ -512,6 +549,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
               context "when that line was changed between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(initial_commit, create_file_commit) }
                 let(:new_diff_refs) { diff_refs(create_file_commit, update_line_commit) }
+                let(:change_diff_refs) { diff_refs(create_file_commit, update_line_commit) }
                 let(:old_position) { position(new_path: file_name, new_line: 2) }
 
                 # old diff:
@@ -525,14 +563,20 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 #   2 + BB
                 # 3 3   C
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: 2,
+                    new_line: nil
+                  )
                 end
               end
 
               context "when that line was deleted between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(initial_commit, move_line_commit) }
                 let(:new_diff_refs) { diff_refs(move_line_commit, delete_line_commit) }
+                let(:change_diff_refs) { diff_refs(move_line_commit, delete_line_commit) }
                 let(:old_position) { position(new_path: file_name, new_line: 3) }
 
                 # old diff:
@@ -545,8 +589,13 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 # 2 2   A
                 # 3   - C
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: 3,
+                    new_line: nil
+                  )
                 end
               end
             end
@@ -558,6 +607,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
             context "when the file's content was unchanged between the old and the new diff" do
               let(:old_diff_refs) { diff_refs(initial_commit, delete_line_commit) }
               let(:new_diff_refs) { diff_refs(delete_line_commit, rename_file_commit) }
+              let(:change_diff_refs) { diff_refs(initial_commit, delete_line_commit) }
               let(:old_position) { position(new_path: file_name, new_line: 2) }
 
               # old diff:
@@ -569,8 +619,13 @@ describe Gitlab::Diff::PositionTracer, lib: true do
               # 1 1   BB
               # 2 2   A
 
-              it "returns nil since the line doesn't exist in the new diffs anymore" do
-                expect(subject).to be_nil
+              it "returns the position of the change" do
+                expect_change_position(
+                  old_path: file_name,
+                  new_path: file_name,
+                  old_line: nil,
+                  new_line: 2
+                )
               end
             end
 
@@ -628,6 +683,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
               context "when that line was changed between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(initial_commit, delete_line_commit) }
                 let(:new_diff_refs) { diff_refs(delete_line_commit, update_line_again_commit) }
+                let(:change_diff_refs) { diff_refs(delete_line_commit, update_line_again_commit) }
                 let(:old_position) { position(new_path: file_name, new_line: 2) }
 
                 # old diff:
@@ -640,28 +696,13 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 # 2   - A
                 #   2 + AA
 
-                it "returns nil" do
-                  expect(subject).to be_nil
-                end
-              end
-
-              context "when that line was deleted between the old and the new diff" do
-                let(:old_diff_refs) { diff_refs(initial_commit, delete_line_commit) }
-                let(:new_diff_refs) { diff_refs(delete_line_commit, delete_line_again_commit) }
-                let(:old_position) { position(new_path: file_name, new_line: 1) }
-
-                # old diff:
-                #   1 + BB
-                #   2 + A
-                #
-                # new diff:
-                # file_name -> new_file_name
-                # 1   - BB
-                # 2   - A
-                #   1 + AA
-
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: new_file_name,
+                    old_line: 2,
+                    new_line: nil
+                  )
                 end
               end
             end
@@ -673,6 +714,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
             context "when the file's content was unchanged between the old and the new diff" do
               let(:old_diff_refs) { diff_refs(initial_commit, delete_line_commit) }
               let(:new_diff_refs) { diff_refs(delete_line_commit, delete_file_commit) }
+              let(:change_diff_refs) { diff_refs(delete_line_commit, delete_file_commit) }
               let(:old_position) { position(new_path: file_name, new_line: 2) }
 
               # old diff:
@@ -683,8 +725,13 @@ describe Gitlab::Diff::PositionTracer, lib: true do
               # 1   - BB
               # 2   - A
 
-              it "returns nil" do
-                expect(subject).to be_nil
+              it "returns the position of the change" do
+                expect_change_position(
+                  old_path: file_name,
+                  new_path: file_name,
+                  old_line: 2,
+                  new_line: nil
+                )
               end
             end
 
@@ -692,6 +739,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
               context "when that line was unchanged between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(initial_commit, move_line_commit) }
                 let(:new_diff_refs) { diff_refs(delete_line_commit, delete_file_commit) }
+                let(:change_diff_refs) { diff_refs(move_line_commit, delete_file_commit) }
                 let(:old_position) { position(new_path: file_name, new_line: 2) }
 
                 # old diff:
@@ -703,14 +751,20 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 # 1   - BB
                 # 2   - A
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: 2,
+                    new_line: nil
+                  )
                 end
               end
 
               context "when that line was moved between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(initial_commit, update_line_commit) }
                 let(:new_diff_refs) { diff_refs(move_line_commit, delete_file_commit) }
+                let(:change_diff_refs) { diff_refs(update_line_commit, delete_file_commit) }
                 let(:old_position) { position(new_path: file_name, new_line: 2) }
 
                 # old diff:
@@ -723,14 +777,20 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 # 2   - A
                 # 3   - C
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: 2,
+                    new_line: nil
+                  )
                 end
               end
 
               context "when that line was changed between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(initial_commit, create_file_commit) }
                 let(:new_diff_refs) { diff_refs(update_line_commit, delete_file_commit) }
+                let(:change_diff_refs) { diff_refs(create_file_commit, delete_file_commit) }
                 let(:old_position) { position(new_path: file_name, new_line: 2) }
 
                 # old diff:
@@ -743,14 +803,20 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 # 2   - BB
                 # 3   - C
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: 2,
+                    new_line: nil
+                  )
                 end
               end
 
               context "when that line was deleted between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(initial_commit, move_line_commit) }
                 let(:new_diff_refs) { diff_refs(delete_line_commit, delete_file_commit) }
+                let(:change_diff_refs) { diff_refs(move_line_commit, delete_file_commit) }
                 let(:old_position) { position(new_path: file_name, new_line: 3) }
 
                 # old diff:
@@ -762,8 +828,13 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 # 1   - BB
                 # 2   - A
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: 3,
+                    new_line: nil
+                  )
                 end
               end
             end
@@ -775,6 +846,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
             context "when the file's content was unchanged between the old and the new diff" do
               let(:old_diff_refs) { diff_refs(initial_commit, create_file_commit) }
               let(:new_diff_refs) { diff_refs(create_file_commit, create_second_file_commit) }
+              let(:change_diff_refs) { diff_refs(initial_commit, create_file_commit) }
               let(:old_position) { position(new_path: file_name, new_line: 2) }
 
               # old diff:
@@ -787,8 +859,13 @@ describe Gitlab::Diff::PositionTracer, lib: true do
               # 2 2   B
               # 3 3   C
 
-              it "returns nil" do
-                expect(subject).to be_nil
+              it "returns the position of the change" do
+                expect_change_position(
+                  old_path: file_name,
+                  new_path: file_name,
+                  old_line: nil,
+                  new_line: 2
+                )
               end
             end
 
@@ -796,6 +873,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
               context "when that line was unchanged between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(initial_commit, create_file_commit) }
                 let(:new_diff_refs) { diff_refs(update_line_commit, update_second_file_line_commit) }
+                let(:change_diff_refs) { diff_refs(initial_commit, update_line_commit) }
                 let(:old_position) { position(new_path: file_name, new_line: 1) }
 
                 # old diff:
@@ -808,14 +886,20 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 # 2 2   BB
                 # 3 3   C
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: nil,
+                    new_line: 1
+                  )
                 end
               end
 
               context "when that line was moved between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(initial_commit, update_line_commit) }
                 let(:new_diff_refs) { diff_refs(move_line_commit, move_second_file_line_commit) }
+                let(:change_diff_refs) { diff_refs(initial_commit, move_line_commit) }
                 let(:old_position) { position(new_path: file_name, new_line: 2) }
 
                 # old diff:
@@ -828,14 +912,20 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 # 2 2   A
                 # 3 3   C
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: nil,
+                    new_line: 1
+                  )
                 end
               end
 
               context "when that line was changed between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(initial_commit, create_file_commit) }
                 let(:new_diff_refs) { diff_refs(update_line_commit, update_second_file_line_commit) }
+                let(:change_diff_refs) { diff_refs(create_file_commit, update_second_file_line_commit) }
                 let(:old_position) { position(new_path: file_name, new_line: 2) }
 
                 # old diff:
@@ -848,14 +938,20 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 # 2 2   BB
                 # 3 3   C
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: 2,
+                    new_line: nil
+                  )
                 end
               end
 
               context "when that line was deleted between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(initial_commit, move_line_commit) }
                 let(:new_diff_refs) { diff_refs(delete_line_commit, delete_second_file_line_commit) }
+                let(:change_diff_refs) { diff_refs(move_line_commit, delete_second_file_line_commit) }
                 let(:old_position) { position(new_path: file_name, new_line: 3) }
 
                 # old diff:
@@ -867,8 +963,13 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 # 1 1   BB
                 # 2 2   A
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: 3,
+                    new_line: nil
+                  )
                 end
               end
             end
@@ -957,6 +1058,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
               context "when that line was changed or deleted between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(create_file_commit, move_line_commit) }
                 let(:new_diff_refs) { diff_refs(initial_commit, create_file_commit) }
+                let(:change_diff_refs) { diff_refs(move_line_commit, create_file_commit) }
                 let(:old_position) { position(old_path: file_name, new_path: file_name, new_line: 1) }
 
                 # old diff:
@@ -970,8 +1072,13 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 #   2 + B
                 #   3 + C
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: 1,
+                    new_line: nil
+                  )
                 end
               end
             end
@@ -980,6 +1087,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
           context "when the position pointed at a deleted line in the old diff" do
             let(:old_diff_refs) { diff_refs(create_file_commit, update_line_commit) }
             let(:new_diff_refs) { diff_refs(initial_commit, update_line_commit) }
+            let(:change_diff_refs) { diff_refs(create_file_commit, initial_commit) }
             let(:old_position) { position(old_path: file_name, new_path: file_name, old_line: 2) }
 
             # old diff:
@@ -993,8 +1101,13 @@ describe Gitlab::Diff::PositionTracer, lib: true do
             #   2 + BB
             #   3 + C
 
-            it "returns nil" do
-              expect(subject).to be_nil
+            it "returns the position of the change" do
+              expect_change_position(
+                old_path: file_name,
+                new_path: file_name,
+                old_line: 2,
+                new_line: nil
+              )
             end
           end
 
@@ -1076,6 +1189,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
               context "when that line was changed or deleted between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(create_file_commit, move_line_commit) }
                 let(:new_diff_refs) { diff_refs(initial_commit, delete_line_commit) }
+                let(:change_diff_refs) { diff_refs(move_line_commit, delete_line_commit) }
                 let(:old_position) { position(old_path: file_name, new_path: file_name, old_line: 3, new_line: 3) }
 
                 # old diff:
@@ -1088,8 +1202,13 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 #   1 + A
                 #   2 + B
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: 3,
+                    new_line: nil
+                  )
                 end
               end
             end
@@ -1182,6 +1301,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
               context "when that line was changed or deleted between the old and the new diff" do
                 let(:old_diff_refs) { diff_refs(create_file_commit, move_line_commit) }
                 let(:new_diff_refs) { diff_refs(create_file_commit, update_line_commit) }
+                let(:change_diff_refs) { diff_refs(move_line_commit, update_line_commit) }
                 let(:old_position) { position(old_path: file_name, new_path: file_name, new_line: 1) }
 
                 # old diff:
@@ -1196,8 +1316,13 @@ describe Gitlab::Diff::PositionTracer, lib: true do
                 #   2 + BB
                 # 3 3   C
 
-                it "returns nil" do
-                  expect(subject).to be_nil
+                it "returns the position of the change" do
+                  expect_change_position(
+                    old_path: file_name,
+                    new_path: file_name,
+                    old_line: 1,
+                    new_line: nil
+                  )
                 end
               end
             end
@@ -1239,7 +1364,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
   describe "typical use scenarios" do
     let(:second_branch_name) { "#{branch_name}-2" }
 
-    def expect_positions(old_attrs, new_attrs)
+    def expect_new_positions(old_attrs, new_attrs)
       old_positions = old_attrs.map do |old_attrs|
         position(old_attrs)
       end
@@ -1248,8 +1373,14 @@ describe Gitlab::Diff::PositionTracer, lib: true do
         position_tracer.trace(old_position)
       end
 
-      new_positions.zip(new_attrs).each do |new_position, new_attrs|
-        expect_new_position(new_attrs, new_position)
+      aggregate_failures do
+        new_positions.zip(new_attrs).each do |new_position, new_attrs|
+          if new_attrs&.delete(:change)
+            expect_change_position(new_attrs, new_position)
+          else
+            expect_new_position(new_attrs, new_position)
+          end
+        end
       end
     end
 
@@ -1330,6 +1461,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
     describe "simple push of new commit" do
       let(:old_diff_refs) { diff_refs(create_file_commit, update_file_commit) }
       let(:new_diff_refs) { diff_refs(create_file_commit, update_file_again_commit) }
+      let(:change_diff_refs) { diff_refs(update_file_commit, update_file_again_commit) }
 
       # old diff:
       # 1 1   A
@@ -1368,14 +1500,14 @@ describe Gitlab::Diff::PositionTracer, lib: true do
           { old_path: file_name, new_path: file_name, old_line: 1, new_line: 1 },
           { old_path: file_name,                      old_line: 2              },
           { old_path: file_name, new_path: file_name, old_line: 3, new_line: 3 },
-          { old_path: file_name,                      old_line: 4, new_line: 4 },
-          nil,
+          {                      new_path: file_name,              new_line: 4,   change: true },
+          {                      new_path: file_name, old_line: 3,                change: true },
           { old_path: file_name, new_path: file_name, old_line: 5, new_line: 5 },
-          { old_path: file_name,                      old_line: 6              },
+          {                      new_path: file_name, old_line: 5,                change: true },
           {                      new_path: file_name,              new_line: 7 }
         ]
 
-        expect_positions(old_position_attrs, new_position_attrs)
+        expect_new_positions(old_position_attrs, new_position_attrs)
       end
     end
 
@@ -1402,6 +1534,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
 
       let(:old_diff_refs) { diff_refs(create_file_commit, update_file_commit) }
       let(:new_diff_refs) { diff_refs(create_file_commit, second_create_file_commit) }
+      let(:change_diff_refs) { diff_refs(update_file_commit, second_create_file_commit) }
 
       # old diff:
       # 1 1   A
@@ -1440,20 +1573,21 @@ describe Gitlab::Diff::PositionTracer, lib: true do
           { old_path: file_name, new_path: file_name, old_line: 1, new_line: 1 },
           { old_path: file_name,                      old_line: 2              },
           { old_path: file_name, new_path: file_name, old_line: 3, new_line: 3 },
-          { old_path: file_name,                      old_line: 4, new_line: 4 },
-          nil,
+          {                      new_path: file_name,              new_line: 4,   change: true },
+          { old_path: file_name,                      old_line: 3,                change: true },
           { old_path: file_name, new_path: file_name, old_line: 5, new_line: 5 },
-          { old_path: file_name,                      old_line: 6              },
+          { old_path: file_name,                      old_line: 5,                change: true },
           {                      new_path: file_name,              new_line: 7 }
         ]
 
-        expect_positions(old_position_attrs, new_position_attrs)
+        expect_new_positions(old_position_attrs, new_position_attrs)
       end
     end
 
     describe "force push to delete last commit" do
       let(:old_diff_refs) { diff_refs(create_file_commit, update_file_again_commit) }
       let(:new_diff_refs) { diff_refs(create_file_commit, update_file_commit) }
+      let(:change_diff_refs) { diff_refs(update_file_again_commit, update_file_commit) }
 
       # old diff:
       # 1 1   A
@@ -1492,16 +1626,16 @@ describe Gitlab::Diff::PositionTracer, lib: true do
         new_position_attrs = [
           { old_path: file_name, new_path: file_name, old_line: 1, new_line: 1 },
           { old_path: file_name,                      old_line: 2              },
-          nil,
+          { old_path: file_name,                      old_line: 2,                change: true },
           { old_path: file_name, new_path: file_name, old_line: 3, new_line: 2 },
-          { old_path: file_name,                      old_line: 4              },
+          { old_path: file_name,                      old_line: 4,                change: true },
           { old_path: file_name, new_path: file_name, old_line: 5, new_line: 4 },
-          { old_path: file_name, new_path: file_name, old_line: 6, new_line: 5 },
-          nil,
+          {                      new_path: file_name,              new_line: 5,   change: true },
+          { old_path: file_name,                      old_line: 6,                change: true },
           {                      new_path: file_name,              new_line: 6 }
         ]
 
-        expect_positions(old_position_attrs, new_position_attrs)
+        expect_new_positions(old_position_attrs, new_position_attrs)
       end
     end
 
@@ -1567,6 +1701,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
 
       let(:old_diff_refs) { diff_refs(create_file_commit, update_file_again_commit) }
       let(:new_diff_refs) { diff_refs(create_file_commit, overwrite_update_file_again_commit) }
+      let(:change_diff_refs) { diff_refs(update_file_again_commit, overwrite_update_file_again_commit) }
 
       # old diff:
       # 1 1   A
@@ -1618,7 +1753,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
           {                      new_path: file_name,              new_line: 10 }, # + G
         ]
 
-        expect_positions(old_position_attrs, new_position_attrs)
+        expect_new_positions(old_position_attrs, new_position_attrs)
       end
     end
 
@@ -1643,6 +1778,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
 
       let(:old_diff_refs) { diff_refs(create_file_commit, update_file_again_commit) }
       let(:new_diff_refs) { diff_refs(create_file_commit, merge_commit) }
+      let(:change_diff_refs) { diff_refs(update_file_again_commit, merge_commit) }
 
       # old diff:
       # 1 1   A
@@ -1694,13 +1830,14 @@ describe Gitlab::Diff::PositionTracer, lib: true do
           {                      new_path: file_name,              new_line: 10 }, # + G
         ]
 
-        expect_positions(old_position_attrs, new_position_attrs)
+        expect_new_positions(old_position_attrs, new_position_attrs)
       end
     end
 
     describe "changing target branch" do
       let(:old_diff_refs) { diff_refs(create_file_commit, update_file_again_commit) }
       let(:new_diff_refs) { diff_refs(update_file_commit, update_file_again_commit) }
+      let(:change_diff_refs) { diff_refs(create_file_commit, update_file_commit) }
 
       # old diff:
       # 1 1   A
@@ -1739,7 +1876,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
 
         new_position_attrs = [
           { old_path: file_name, new_path: file_name, old_line: 1, new_line: 1 },
-          nil,
+          { old_path: file_name,                      old_line: 2,                change: true },
           {                      new_path: file_name,              new_line: 2 },
           { old_path: file_name, new_path: file_name, old_line: 2, new_line: 3 },
           {                      new_path: file_name,              new_line: 4 },
@@ -1749,7 +1886,7 @@ describe Gitlab::Diff::PositionTracer, lib: true do
           {                      new_path: file_name,              new_line: 7 }
         ]
 
-        expect_positions(old_position_attrs, new_position_attrs)
+        expect_new_positions(old_position_attrs, new_position_attrs)
       end
     end
   end
diff --git a/spec/lib/gitlab/import_export/safe_model_attributes.yml b/spec/lib/gitlab/import_export/safe_model_attributes.yml
index c22fba11225..96054c996fd 100644
--- a/spec/lib/gitlab/import_export/safe_model_attributes.yml
+++ b/spec/lib/gitlab/import_export/safe_model_attributes.yml
@@ -54,6 +54,7 @@ Note:
 - type
 - position
 - original_position
+- change_position
 - resolved_at
 - resolved_by_id
 - discussion_id
diff --git a/spec/models/concerns/discussion_on_diff_spec.rb b/spec/models/concerns/discussion_on_diff_spec.rb
index 8571e85627c..f3e148f95f0 100644
--- a/spec/models/concerns/discussion_on_diff_spec.rb
+++ b/spec/models/concerns/discussion_on_diff_spec.rb
@@ -21,4 +21,30 @@ describe DiscussionOnDiff, model: true do
       end
     end
   end
+
+  describe '#line_code_in_diffs' do
+    context 'when the discussion is active in the diff' do
+      let(:diff_refs) { subject.position.diff_refs }
+
+      it 'returns the current line code' do
+        expect(subject.line_code_in_diffs(diff_refs)).to eq(subject.line_code)
+      end
+    end
+
+    context 'when the discussion was created in the diff' do
+      let(:diff_refs) { subject.original_position.diff_refs }
+
+      it 'returns the original line code' do
+        expect(subject.line_code_in_diffs(diff_refs)).to eq(subject.original_line_code)
+      end
+    end
+
+    context 'when the discussion is unrelated to the diff' do
+      let(:diff_refs) { subject.project.commit(RepoHelpers.sample_commit.id).diff_refs }
+
+      it 'returns nil' do
+        expect(subject.line_code_in_diffs(diff_refs)).to be_nil
+      end
+    end
+  end
 end
diff --git a/spec/models/diff_discussion_spec.rb b/spec/models/diff_discussion_spec.rb
index 81f338745b1..45b2f6e4beb 100644
--- a/spec/models/diff_discussion_spec.rb
+++ b/spec/models/diff_discussion_spec.rb
@@ -48,7 +48,7 @@ describe DiffDiscussion, model: true do
       end
 
       it 'returns the diff ID for the version to show' do
-        expect(diff_id: merge_request_diff1.id)
+        expect(subject.merge_request_version_params).to eq(diff_id: merge_request_diff1.id)
       end
     end
 
@@ -65,6 +65,11 @@ describe DiffDiscussion, model: true do
 
       let(:diff_note) { create(:diff_note_on_merge_request, noteable: merge_request, project: project, position: position) }
 
+      before do
+        diff_note.position = diff_note.original_position
+        diff_note.save!
+      end
+
       it 'returns the diff ID and start sha of the versions to compare' do
         expect(subject.merge_request_version_params).to eq(diff_id: merge_request_diff3.id, start_sha: merge_request_diff1.head_commit_sha)
       end
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index 0e05f719499..da915c49d3c 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -1213,7 +1213,7 @@ describe MergeRequest, models: true do
 
       expect(Notes::DiffPositionUpdateService).to receive(:new).with(
         subject.project,
-        nil,
+        subject.author,
         old_diff_refs: old_diff_refs,
         new_diff_refs: commit.diff_refs,
         paths: note.position.paths
@@ -1222,7 +1222,7 @@ describe MergeRequest, models: true do
       expect_any_instance_of(Notes::DiffPositionUpdateService).to receive(:execute).with(note)
       expect_any_instance_of(DiffNote).to receive(:save).once
 
-      subject.reload_diff
+      subject.reload_diff(subject.author)
     end
   end
 
@@ -1534,4 +1534,36 @@ describe MergeRequest, models: true do
       end
     end
   end
+
+  describe '#version_params_for' do
+    subject { create(:merge_request, importing: true) }
+    let(:project) { subject.project }
+    let!(:merge_request_diff1) { subject.merge_request_diffs.create(head_commit_sha: '6f6d7e7ed97bb5f0054f2b1df789b39ca89b6ff9') }
+    let!(:merge_request_diff2) { subject.merge_request_diffs.create(head_commit_sha: nil) }
+    let!(:merge_request_diff3) { subject.merge_request_diffs.create(head_commit_sha: '5937ac0a7beb003549fc5fd26fc247adbce4a52e') }
+
+    context 'when the diff refs are for an older merge request version' do
+      let(:diff_refs) { merge_request_diff1.diff_refs }
+
+      it 'returns the diff ID for the version to show' do
+        expect(subject.version_params_for(diff_refs)).to eq(diff_id: merge_request_diff1.id)
+      end
+    end
+
+    context 'when the diff refs are for a comparison between merge request versions' do
+      let(:diff_refs) { merge_request_diff3.compare_with(merge_request_diff1.head_commit_sha).diff_refs }
+
+      it 'returns the diff ID and start sha of the versions to compare' do
+        expect(subject.version_params_for(diff_refs)).to eq(diff_id: merge_request_diff3.id, start_sha: merge_request_diff1.head_commit_sha)
+      end
+    end
+
+    context 'when the diff refs are not for a merge request version' do
+      let(:diff_refs) { project.commit(sample_commit.id).diff_refs }
+
+      it 'returns nil' do
+        expect(subject.version_params_for(diff_refs)).to be_nil
+      end
+    end
+  end
 end
diff --git a/spec/services/notes/diff_position_update_service_spec.rb b/spec/services/notes/diff_position_update_service_spec.rb
index d73ae51fbc3..380c296fd3a 100644
--- a/spec/services/notes/diff_position_update_service_spec.rb
+++ b/spec/services/notes/diff_position_update_service_spec.rb
@@ -2,6 +2,7 @@ require 'spec_helper'
 
 describe Notes::DiffPositionUpdateService, services: true do
   let(:project) { create(:project, :repository) }
+  let(:current_user) { project.owner }
   let(:create_commit) { project.commit("913c66a37b4a45b9769037c55c2d238bd0942d2e") }
   let(:modify_commit) { project.commit("874797c3a73b60d2187ed6e2fcabd289ff75171e") }
   let(:edit_commit) { project.commit("570e7b2abdd848b95f2f578043fc23bd6f6fd24d") }
@@ -25,7 +26,7 @@ describe Notes::DiffPositionUpdateService, services: true do
   subject do
     described_class.new(
       project,
-      nil,
+      current_user,
       old_diff_refs: old_diff_refs,
       new_diff_refs: new_diff_refs,
       paths: [path]
@@ -170,6 +171,23 @@ describe Notes::DiffPositionUpdateService, services: true do
         expect(note.original_position).to eq(old_position)
         expect(note.position).to eq(old_position)
       end
+
+      it 'sets the change position' do
+        subject.execute(note)
+
+        change_position = note.change_position
+        expect(change_position.start_sha).to eq(old_diff_refs.head_sha)
+        expect(change_position.head_sha).to eq(new_diff_refs.head_sha)
+        expect(change_position.old_line).to eq(9)
+        expect(change_position.new_line).to be_nil
+      end
+
+      it 'creates a system note' do
+        expect(SystemNoteService).to receive(:diff_discussion_outdated).with(
+          note.to_discussion, project, current_user, instance_of(Gitlab::Diff::Position))
+
+        subject.execute(note)
+      end
     end
   end
 end
diff --git a/spec/services/system_note_service_spec.rb b/spec/services/system_note_service_spec.rb
index 7a9cd7553b1..5a7cfaff7fb 100644
--- a/spec/services/system_note_service_spec.rb
+++ b/spec/services/system_note_service_spec.rb
@@ -1034,4 +1034,35 @@ describe SystemNoteService, services: true do
       expect(subject.note).to eq 'resolved all discussions'
     end
   end
+
+  describe '.diff_discussion_outdated' do
+    let(:discussion) { create(:diff_note_on_merge_request).to_discussion }
+    let(:merge_request) { discussion.noteable }
+    let(:project) { merge_request.source_project }
+    let(:change_position) { discussion.position }
+
+    def reloaded_merge_request
+      MergeRequest.find(merge_request.id)
+    end
+
+    subject { described_class.diff_discussion_outdated(discussion, project, author, change_position) }
+
+    it_behaves_like 'a system note' do
+      let(:expected_noteable) { discussion.first_note.noteable }
+      let(:action)            { 'outdated' }
+    end
+
+    it 'creates a new note in the discussion' do
+      # we need to completely rebuild the merge request object, or the `@discussions` on the merge request are not reloaded.
+      expect { subject }.to change { reloaded_merge_request.discussions.first.notes.size }.by(1)
+    end
+
+    it 'links to the diff in the system note' do
+      expect(subject.note).to include('version 1')
+
+      diff_id = merge_request.merge_request_diff.id
+      line_code = change_position.line_code(project.repository)
+      expect(subject.note).to include(diffs_namespace_project_merge_request_url(project.namespace, project, merge_request, diff_id: diff_id, anchor: line_code))
+    end
+  end
 end