[ruby-core:118617] [Ruby master Bug#20637] SyntaxError class definition in method body can be bypassed

16 Jul 2024

      Issue #20637 has been updated by zverok (Victor Shepelev).

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

```ruby
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
```

----------------------------------------
Bug #20637: SyntaxError class definition in method body can be bypassed
https://bugs.ruby-lang.org/issues/20637#change-109145

* Author: tompng (tomoya ishida)
* Status: Open
* 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
----------------------------------------
Class definition in method body is prohibited in Ruby
~~~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 <<`
~~~ruby
def f
  class << Object.new
    class ::A; end # Syntax OK
    module B; end # Syntax OK
  end
end
~~~

-- 
https://bugs.ruby-lang.org/