Allow to fork to the same namespace with different name

API allows to set alternative name and path for forks,
so same namespace as source project can be valid fork target

app/controllers/projects/forks_controller.rb

@@ -39,7 +39,7 @@ class Projects::ForksController < Projects::ApplicationController
   # rubocop: enable CodeReuse/ActiveRecord
 
   def new
-    @namespaces = fork_service.valid_fork_targets
+    @namespaces = fork_service.valid_fork_targets - [project.namespace]
   end
 
   # rubocop: disable CodeReuse/ActiveRecord

app/finders/fork_targets_finder.rb

@@ -8,7 +8,7 @@ class ForkTargetsFinder
 
   # rubocop: disable CodeReuse/ActiveRecord
   def execute
-    ::Namespace.where(id: user.manageable_namespaces).where.not(id: project.namespace).sort_by_type
+    ::Namespace.where(id: user.manageable_namespaces).sort_by_type
   end
   # rubocop: enable CodeReuse/ActiveRecord
 

changelogs/unreleased/208167-bigfix-unable-to-fork-project-to-the-same-namespace.yml

+---
+title: Allow to fork to the same namespace and different path via API call
+merge_request: 26062
+author:
+type: fixed

spec/finders/fork_targets_finder_spec.rb

@@ -28,8 +28,8 @@ describe ForkTargetsFinder do
   end
 
   describe '#execute' do
-    it 'returns all user manageable namespaces except project namespace' do
-      expect(finder.execute).to match_array([user.namespace, maintained_group, owned_group])
+    it 'returns all user manageable namespaces' do
+      expect(finder.execute).to match_array([user.namespace, maintained_group, owned_group, project.namespace])
     end
   end
 end

spec/requests/api/projects_spec.rb

@@ -2935,6 +2935,26 @@ describe API::Projects do
         expect(response).to have_gitlab_http_status(:conflict)
         expect(json_response['message']['name']).to eq(['has already been taken'])
       end
+
+      it 'forks to the same namespace with alternative path and name' do
+        post api("/projects/#{project.id}/fork", user), params: { path: 'path_2', name: 'name_2' }
+
+        expect(response).to have_gitlab_http_status(:created)
+        expect(json_response['name']).to eq('name_2')
+        expect(json_response['path']).to eq('path_2')
+        expect(json_response['owner']['id']).to eq(user.id)
+        expect(json_response['namespace']['id']).to eq(user.namespace.id)
+        expect(json_response['forked_from_project']['id']).to eq(project.id)
+        expect(json_response['import_status']).to eq('scheduled')
+      end
+
+      it 'fails to fork to the same namespace without alternative path and name' do
+        post api("/projects/#{project.id}/fork", user)
+
+        expect(response).to have_gitlab_http_status(:conflict)
+        expect(json_response['message']['path']).to eq(['has already been taken'])
+        expect(json_response['message']['name']).to eq(['has already been taken'])
+      end
     end
 
     context 'when unauthenticated' do