Issue #19270 has been updated by 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`: ```ruby class A C = 1 end a = A.new klass = (class << a; self; end) p klass::C # => 1 ``` ---------------------------------------- Bug #19270: Constants lookup and a singleton class issue https://bugs.ruby-lang.org/issues/19270#change-100876 * Author: andrykonchin (Andrew Konchin) * Status: Open * Priority: Normal * ruby -v: 3.1.3 * Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN ---------------------------------------- I've noticed that a constant declared in a singleton class may be not visible on an object: ```ruby 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? -- https://bugs.ruby-lang.org/