Default dependency job stage index to Infinity, and correctly report it as...

Default dependency job stage index to Infinity, and correctly report it as undefined in prior stages

changelogs/unreleased/report-missing-job-dependency.yml

+---
+title: Default dependency job stage index to Infinity, and correctly report it as
+  undefined in prior stages
+merge_request: 31116
+author:
+type: fixed

lib/gitlab/ci/yaml_processor.rb

@@ -144,7 +144,9 @@ module Gitlab
         job[:dependencies].each do |dependency|
           raise ValidationError, "#{name} job: undefined dependency: #{dependency}" unless @jobs[dependency.to_sym]
-          unless @stages.index(@jobs[dependency.to_sym][:stage]) < stage_index
+          dependency_stage_index = @stages.index(@jobs[dependency.to_sym][:stage])
+
+          unless dependency_stage_index.present? && dependency_stage_index < stage_index
             raise ValidationError, "#{name} job: dependency #{dependency} is not defined in prior stages"
           end
         end

spec/lib/gitlab/ci/yaml_processor_spec.rb

@@ -1085,6 +1085,31 @@ module Gitlab
           it { expect { subject }.to raise_error(Gitlab::Ci::YamlProcessor::ValidationError, 'test1 job: dependency deploy is not defined in prior stages') }
         end
+
+        context 'when a job depends on another job that references a not-yet defined stage' do
+          let(:config) do
+            {
+              "stages" => [
+                "version"
+              ],
+              "version" => {
+                "stage" => "version",
+                "dependencies" => ["release:components:versioning"],
+                "script" => ["./versioning/versioning"]
+              },
+              ".release_go" => {
+                "stage" => "build",
+                "script" => ["cd versioning"]
+              },
+              "release:components:versioning" => {
+                "stage" => "build",
+                "script" => ["cd versioning"]
+              }
+            }
+          end
+
+          it { expect { subject }.to raise_error(Gitlab::Ci::YamlProcessor::ValidationError, /is not defined in prior stages/) }
+        end
       end
       describe "Hidden jobs" do