[ruby-core:80835] [Ruby trunk Bug#13498] Weakref, Weakmap and define_finalizer don't work on frozen objects

Issue #13498 has been reported by herwin (Herwin W).

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Bug #13498: Weakref, Weakmap and define_finalizer don't work on frozen objects
https://bugs.ruby-lang.org/issues/13498

* Author: herwin (Herwin W)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux] (but seen the same issue with 2.3 and 2.1)
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
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I'm just creating a single ticket for these issues, I guess they're actually all the same (I've seen weakref uses weakmap, not sure about define_finalizer).

```ruby
require 'weakref'

map = ObjectSpace::WeakMap.new

o = Object.new
o.freeze

begin
  WeakRef.new(o)
rescue => e
  STDERR.puts e
end

begin
  map[o] = 'foo'
rescue => e
  STDERR.puts e
end

begin
  map['bar'] = o
rescue => e
  STDERR.puts e
end

begin
  ObjectSpace.define_finalizer(o, ->(id) { p id })
rescue => e
  STDERR.puts e
end
```

Every statement here raises the runtime error "can't modify frozen Object". The documentation doesn't mention that frozen objects are not allowed, the closest reference we get is a short paragraph in WeakRef: "With this you will have to limit your self to String keys, otherwise you will get an ArgumentError because WeakRef cannot create a finalizer for a Symbol. Symbols are immutable and cannot be garbage collected"



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