Bug #20637: SyntaxError class definition in method body can be bypassed

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: ruby 3.4.0dev (2024-07-11T06:59:45Z master a1f7432550) [x86_64-linux]
• Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN

[tompng (tomoya ishida)]

Class definition in method body is prohibited in Ruby

def f
  class ::A; end # class definition in method body (SyntaxError)
  module B; end # module definition in method body (SyntaxError)
end

But it can be bypassed by using class <<

def f
  class << Object.new
    class ::A; end # Syntax OK
    module B; end # Syntax OK
  end
end

[zverok (Victor Shepelev)]

As far as I understand, that’s legitimate code that works and might be useful for some metaprogramming:

def f(o)
  class << o
    class ::A; end # Syntax OK
    module B; end # Syntax OK
  end
end

o = Object.new
f(o)
o.singleton_class::B #=> #<Class:0x00007efd52a7bba0>::B
A #=> A

[tompng (tomoya ishida)]

dynamic constant assignment (SyntaxError) can be also bypassed

def f
  class << Object.new
    ::A = 1
  end
end