Merge #3661: [go_modules] Gracefully handle +incompatible versions wh…

…en checking for updates

go_modules/lib/dependabot/go_modules/update_checker/latest_version_finder.rb

@@ -2,6 +2,7 @@
 
 require "excon"
 
+require "dependabot/go_modules/update_checker"
 require "dependabot/shared_helpers"
 require "dependabot/errors"
 require "dependabot/go_modules/requirement"
@@ -70,6 +71,8 @@ def available_versions
                 }
               )
 
+              return [version_class.new(dependency.version)] if version_strings.nil?
+
               version_strings.select { |v| version_class.correct?(v) }.
                 map { |v| version_class.new(v) }
             end

go_modules/spec/dependabot/go_modules/update_checker/latest_version_finder_spec.rb

@@ -0,0 +1,65 @@
+# frozen_string_literal: true
+
+require "dependabot/dependency"
+require "dependabot/dependency_file"
+require "dependabot/go_modules/native_helpers"
+require "dependabot/go_modules/update_checker/latest_version_finder"
+
+RSpec.describe Dependabot::GoModules::UpdateChecker::LatestVersionFinder do
+  describe "#latest_version" do
+    let(:dependency_name) { "github.com/dependabot-fixtures/go-modules-lib" }
+
+    let(:dependency_version) { "1.0.0" }
+
+    let(:dependency) do
+      Dependabot::Dependency.new(
+        name: dependency_name,
+        version: dependency_version,
+        package_manager: "go_modules",
+        requirements: [{
+          file: "go.mod",
+          requirement: dependency_version,
+          groups: [],
+          source: { type: "default", source: dependency_name }
+        }]
+      )
+    end
+
+    let(:go_mod_content) do
+      <<~GOMOD
+        module foobar
+        require #{dependency_name} v#{dependency_version}
+      GOMOD
+    end
+
+    let(:dependency_files) do
+      [
+        Dependabot::DependencyFile.new(
+          name: "go.mod",
+          content: go_mod_content
+        )
+      ]
+    end
+
+    context "when the latest version is an '+incompatible' version" do # https://golang.org/ref/mod#incompatible-versions
+      let(:dependency_name) { "github.com/dependabot-fixtures/go-modules-incompatible" }
+      let(:dependency_version) { "2.0.0+incompatible" }
+
+      it "returns the current version" do
+        finder = described_class.new(
+          dependency: dependency,
+          dependency_files: dependency_files,
+          credentials: [{
+            "type" => "git_source",
+            "host" => "github.com",
+            "username" => "x-access-token",
+            "password" => "token"
+          }],
+          ignored_versions: []
+        )
+
+        expect(finder.latest_version).to eq(Dependabot::GoModules::Version.new("2.0.0+incompatible"))
+      end
+    end
+  end
+end