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: { test: false }
+      """
+    And a file named "source/index.erb" with:
+      """
+      <%= foo %>
+      """
+    Given a built app at "empty_app"
+    Then the exit status should be 1

middleman-core/lib/middleman-core/template_context.rb

@@ -34,6 +34,18 @@ module Middleman
       @app = app
       @locs = locs
       @opts = opts
+
+      @locs.each do |k, _|
+        if self.respond_to?(k)
+          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
+            logger.error(msg)
+          end
+        end
+      end
     end
 
     # Return the current buffer to the caller and clear the value internally.