Bug #19270: Constants lookup and a singleton class issue

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: 3.1.3
• Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN

[andrykonchin (Andrew Konchin)]

I've noticed that a constant declared in a singleton class may be not visible on an object:

class A
  def c; C; end
end

a = A.new
klass = (class << a; self; end)
klass.const_set(:C, 1)
a.c
# (irb):2:in `c': uninitialized constant A::C (NameError)

I would expect that such constant is visible and accessible on an object. It is expected and intentional behaviour?

[nobu (Nobuyoshi Nakada)]

• Is duplicate of Bug #19269: Constant lookup and #instance_eval added

[alanwu (Alan Wu)]

It's expected behavior. When you have a singleton class singleton_class.ancestors is:

[singleton_class, attached_object_class, ...]

In this case the lookup for starts at A, which is attached_object_class.
It goes to the right so it never searches singleton_class. The lookup resolves when the
constant is on attached_object_class and it starts at singleton_class:

class A
  C = 1
end

a = A.new
klass = (class << a; self; end)
p klass::C # => 1

[Eregon (Benoit Daloze)]

• Status changed from Open to Closed

@alanwu (Alan Wu) By attached_object_class you mean a.class so A?

Constant lookup behaves lexically, here the scopes around method c are A and top-level/Object.
So here it looks in A, then Object, then ancestors of A. It never looks in subclasses of A (and klass is a subclass of A).