better error message for constants autoloaded from anonymous modules [fixes #13204]

load_missing_constant is a private method that basically plays the role of const_missing.
This method has an error condition that is surprising: it raises if the class or module
already has the missing constant. How is it possible that if the class of module has
the constant Ruby has called const_missing in the first place?

The answer is that the from_mod argument is self except for anonymous modules, because
const_missing passes down Object in such case (see the comment in the source code of the
patch for the rationale).

But then, it is better to pass down Object *if Object is also missing the constant* and
otherwise err with an informative message right away.

1 files changed, 13 insertions, 7 deletions

diff --git a/activesupport/lib/active_support/dependencies.rb b/activesupport/lib/active_support/dependencies.rb
index 19d4ff51d7..6be19771f5 100644
--- a/activesupport/lib/active_support/dependencies.rb
+++ b/activesupport/lib/active_support/dependencies.rb
@@ -176,14 +176,22 @@ module ActiveSupport #:nodoc:
       end
 
       def const_missing(const_name)
-        # The interpreter does not pass nesting information, and in the
-        # case of anonymous modules we cannot even make the trade-off of
-        # assuming their name reflects the nesting. Resort to Object as
-        # the only meaningful guess we can make.
-        from_mod = anonymous? ? ::Object : self
+        from_mod = anonymous? ? guess_for_anonymous(const_name) : self
         Dependencies.load_missing_constant(from_mod, const_name)
       end
 
+      # Dependencies assumes the name of the module reflects the nesting (unless
+      # it can be proven that is not the case), and the path to the file that
+      # defines the constant. Anonymous modules cannot follow these conventions
+      # and we assume therefore the user wants to refer to a top-level constant.
+      def guess_for_anonymous(const_name)
+        if Object.const_defined?(const_name)
+          raise NameError, "#{const_name} cannot be autoloaded from an anonymous class or module"
+        else
+          Object
+        end
+      end
+
       def unloadable(const_desc = self)
         super(const_desc)
       end
@@ -456,8 +464,6 @@ module ActiveSupport #:nodoc:
         raise ArgumentError, "A copy of #{from_mod} has been removed from the module tree but is still active!"
       end
 
-      raise NameError, "#{from_mod} is not missing constant #{const_name}!" if from_mod.const_defined?(const_name, false)
-
       qualified_name = qualified_name_for from_mod, const_name
       path_suffix = qualified_name.underscore