Detect Yarn the same way the installer does

CHANGELOG.md

@@ -5,6 +5,7 @@ main
   ember:heroku`. Rails serves the twelve-factor behaviour the gem backported
   natively since `5.0`, and every Rails version this gem supports is `>= 5.2`.
   To upgrade, remove `rails_12factor` from your project's `Gemfile`
+* Replace `EmberCli::App#yarn_enabled?` with `EmberCli::App#yarn?`
 
 0.13.2
 ------

lib/ember_cli/app.rb

@@ -91,8 +91,8 @@ def mountable?
       deploy.mountable?
     end
 
-    def yarn_enabled?
-      options.fetch(:yarn, false)
+    def yarn?
+      paths.yarn?
     end
 
     def bower?

lib/ember_cli/path_set.rb

@@ -134,6 +134,10 @@ def node_modules
       @node_modules ||= root.join("node_modules")
     end
 
+    def yarn?
+      app_options[:yarn].present? || app_options[:yarn_path].present?
+    end
+
     def tee
       @tee ||= path_for_executable("tee")
     end
@@ -169,10 +173,6 @@ def package_manager
       end
     end
 
-    def yarn?
-      app_options[:yarn] || app_options[:yarn_path]
-    end
-
     def app_name
       app.name
     end

lib/generators/ember/heroku/heroku_generator.rb

@@ -9,7 +9,7 @@ def copy_package_json_file
     end
 
     def identify_as_yarn_project
-      if EmberCli.any?(&:yarn_enabled?)
+      if EmberCli.any?(&:yarn?)
         template "yarn.lock.erb", "yarn.lock"
       end
     end

spec/lib/ember_cli/app_spec.rb

@@ -33,24 +33,36 @@
     end
   end
 
-  describe "yarn_enabled?" do
+  describe "#yarn?" do
     context "when configured with yarn: true" do
       it "returns true" do
         app = EmberCli::App.new("with-yarn", yarn: true)
 
-        yarn_enabled = app.yarn_enabled?
-
-        expect(yarn_enabled).to be true
+        expect(app.yarn?).to be true
       end
     end
 
     context "when configured with yarn: false" do
       it "returns false" do
         app = EmberCli::App.new("without-yarn", yarn: false)
 
-        yarn_enabled = app.yarn_enabled?
+        expect(app.yarn?).to be false
+      end
+    end
+
+    context "when configured with yarn_path alone" do
+      it "returns true, matching the executable the installer runs" do
+        app = EmberCli::App.new("with-yarn-path", yarn_path: "/usr/bin/yarn")
+
+        expect(app.yarn?).to be true
+      end
+    end
+
+    context "when configured with neither" do
+      it "returns false" do
+        app = EmberCli::App.new("without-yarn")
 
-        expect(yarn_enabled).to be false
+        expect(app.yarn?).to be false
       end
     end
   end

spec/lib/ember_cli/path_set_spec.rb

@@ -212,6 +212,28 @@
     end
   end
 
+  describe "#yarn?" do
+    it "is true when yarn is requested" do
+      app = build_app(options: { yarn: true })
+      path_set = build_path_set(app: app)
+
+      expect(path_set).to be_yarn
+    end
+
+    it "is true when only the yarn executable is named" do
+      app = build_app(options: { yarn_path: "/usr/bin/yarn" })
+      path_set = build_path_set(app: app)
+
+      expect(path_set).to be_yarn
+    end
+
+    it "is false when yarn is neither requested nor named" do
+      path_set = build_path_set
+
+      expect(path_set).not_to be_yarn
+    end
+  end
+
   describe "#yarn" do
     it "can be overridden" do
       fake_yarn = create_executable(ember_cli_root.join("yarn"))