[ruby-core:121820] [Ruby Bug#21267] respond_to check in Class#allocate is inconsistent

Issue #21267 has been updated by jhawthorn (John Hawthorn).


At RubyKaigi @byroot and I looked at this and came up with a more efficient and resilient way to do the check (https://github.com/ruby/ruby/commit/a6365cef1084191f992ee55fa661cb9aa2196c39). But coming back to this I again don't think it's effective enough to prevent uninitialized values and we should just remove the check.

Again, this check is trying to avoid users being able to make an uninitialized value of these classes, even if they are trying very hard
``` ruby
>> Class.instance_method(:allocate).bind_call(MatchData)
(irb):6:in 'Class#allocate': calling MatchData.allocate is prohibited (TypeError)
```

However we aren't protecting `new` with this check, and can get uninitialized values that way (even though it has been undefined in the same way as `allocate`):

``` ruby
>> Class.instance_method(:new).bind_call(MatchData)
An error occurred when inspecting the object: #<TypeError: uninitialized MatchData>
Result of Kernel#inspect: #<MatchData:0x00007387517150b0>
```

We thought the reason for this special check rather than using `rb_undef_alloc_func` is so that `dup`/`clone` can still work, but that itself is problematic and a way users can get an uninitialized value.

``` ruby
>> match = "a".match(/./)
=> #<MatchData "a">
>> match.clone # wanting this to work is why we don't rb_undef_alloc_func (I think)
=> #<MatchData "a">
>> def match.initialize_copy(x); end
=> :initialize_copy
>> match.clone # now this makes an uninitialized object
An error occurred when inspecting the object: #<TypeError: uninitialized MatchData>
Result of Kernel#inspect: #<MatchData:0x0000738752cfb370>
```

So I am back to believing we should remove the `rb_obj_respond_to` check, because it doesn't provide any safety. It seems like our only options are to tolerate uninitialized objects or use rb_undef_alloc_func.

----------------------------------------
Bug #21267: respond_to check in Class#allocate is inconsistent
https://bugs.ruby-lang.org/issues/21267#change-112883

* Author: jhawthorn (John Hawthorn)
* Status: Open
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
`Class#allocate` has an additional `rb_obj_respond_to(klass, rb_intern("allocate"))` check to forbid allocate being called on a class where it has been made private or undefined, even if used via ex. `bind_call`.

```
>> Rational.allocate
(irb):1:in '<main>': undefined method 'allocate' for class Rational (NoMethodError)
>> Class.instance_method(:allocate).bind_call(Rational)
(irb):1:in 'Class#allocate': calling Rational.allocate is prohibited (TypeError)
```

However I don't think this provides any additional protection from users accessing an uninitialized object, as the user can redefine allocate to anything to bypass the check:

```
>> Class.instance_method(:allocate).bind_call(Class.new(Rational) {def self.allocate; end})
=> (0/1)
```

Or even override `respond_to_missing?`

```
>> Class.instance_method(:allocate).bind_call(Class.new(Rational) {def self.respond_to_missing? *; true; end})
=> (0/1)
```

So I think we should remove this check. For classes that we need to forbid allocation we should use `rb_undef_alloc_func`.

The classes I see this used for are:
* MatchData
* Refinement
* Module
* Complex
* Rational

My main motivation is that this check makes `Class#allocate` slow. There are ways we could improve that, but I don't think the check as-is is useful. If there's an alternative, more robust, check we'd like to do instead of simply removing this I'd be happy to implement it.

This makes `allocate` ~75% faster.

```
|allocate_no_params            |     19.009M|   20.087M|
|                              |           -|     1.06x|
|allocate_allocate             |     20.587M|   35.882M|
|                              |           -|     1.74x|
```

https://github.com/ruby/ruby/pull/13116



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