Consider a dangling reference in the plugin index as not installed

bundler/lib/bundler/plugin.rb

@@ -241,7 +241,9 @@ def hook(event, *args, &arg_blk)
 
     # @return [String, nil] installed path
     def installed?(plugin)
-      index.installed?(plugin)
+      (path = index.installed?(plugin)) &&
+        index.plugin_path(plugin).join(PLUGIN_FILE_NAME).file? &&
+        path
     end
 
     # @return [true, false] whether the plugin is loaded

bundler/spec/plugins/install_spec.rb

@@ -505,6 +505,27 @@ def exec(command, args)
       bundle "exec rake -T", :raise_on_error => false
       expect(err).not_to include("Plugin foo (>= 0) is not installed")
     end
+
+    it "fails bundle commands gracefully when a plugin index reference is left dangling" do
+      build_lib "ga-plugin" do |s|
+        s.write "plugins.rb"
+      end
+
+      install_gemfile <<-G
+        source "#{file_uri_for(gem_repo1)}"
+        plugin 'ga-plugin', :path => "#{lib_path("ga-plugin-1.0")}"
+      G
+
+      expect(out).to include("Installed plugin ga-plugin")
+      plugin_should_be_installed("ga-plugin")
+
+      FileUtils.rm_rf(lib_path("ga-plugin-1.0"))
+
+      plugin_should_not_be_installed("ga-plugin")
+
+      bundle "check", :raise_on_error => false
+      expect(err).to include("Plugin ga-plugin (>= 0) is not installed")
+    end
   end
 
   context "inline gemfiles" do