Throw when trying to overwrite a template context value. Fixes #1884

middleman-core/features/template-key-collision.feature

@@ -0,0 +1,26 @@
+Feature: Don't allow template locals to overwrite template helpers
+
+  Scenario: Normal Template
+    Given an empty app
+    And a file named "config.rb" with:
+      """
+      class TestExt < ::Middleman::Extension
+        expose_to_template foo: :foo
+
+        def foo
+          "bar"
+        end
+      end
+
+      ::Middleman::Extensions.register :test, TestExt
+
+      activate :test
+
+      page "/index.html", locals: { foo: false }
+      """
+    And a file named "source/index.html.erb" with:
+      """
+      <%= foo %>
+      """
+    Given a built app at "empty_app"
+    Then the exit status should be 1

middleman-core/lib/middleman-core/template_renderer.rb

@@ -133,8 +133,20 @@ module Middleman
       # Add extension helpers to context.
       @app.extensions.add_exposed_to_context(context)
 
+      locals.each do |k, _|
+        next unless context.respond_to?(k) && k != :current_path
+
+        msg = "Template local `#{k}` tried to overwrite an existing context value. Please renamed the key when passing to `locals`"
+
+        if @app.build?
+          throw msg
+        else
+          @app.logger.error(msg)
+        end
+      end
+
       content = ::Middleman::Util.instrument 'builder.output.resource.render-template', path: File.basename(path) do
-        _render_with_all_renderers(path, locs, context, opts, &block)
+        _render_with_all_renderers(path, locals, context, options, &block)
       end
 
       # If we need a layout and have a layout, use it