[ruby-core:75336] [Ruby trunk Bug#12344] check_funcall_respond_to behavior seems incorrect

Issue #12344 has been reported by Matt Bedalov.

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Bug #12344: check_funcall_respond_to behavior seems incorrect
https://bugs.ruby-lang.org/issues/12344

* Author: Matt Bedalov
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-linux]
* Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
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Prior to ruby 2.3, and specifically I believe prior to the implementation of this change:
> Thu Aug 20 14:13:27 2015 Nobuyoshi Nakada <[e-mail]>
> vm_eval.c (check_funcall_respond_to)
> share the behavior with rb_obj_respond_to. [ruby-core:70460] [Bug #11465]
> 
> vm_method.c (vm_respond_to)
> extract from rb_obj_respond_to and merge r39881.

When a global method_missing was defined, I believe that "check_funcall_respond_to" would not yield that the to_hash method would be defined for objects of type String.  In Ruby 2.3, it behaves as if to_hash is defined on a String just due to the fact that we've added a global method_missing.  This is hugely problematic when calling a gsub as the second parameter is checked (in str_gsub - string.c:4593) to see if it will act like a hash - i.e. is it a Hash or does it have a to_hash method.  It appears that eventually vm_respond_to() calls METHOD_ENTRY_BASIC() which then makes the false statement that the object responds to the to_hash method.

I believe that a possible fix is to have vm_eval.c - check_funcall_respond_to be changed from:
~~~
static int
check_funcall_respond_to(rb_thread_t *th, VALUE klass, VALUE recv, ID mid)
{
    return vm_respond_to(th, klass, recv, mid, TRUE);
}
~~~


**to**:

~~~
static int
check_funcall_respond_to(rb_thread_t *th, VALUE klass, VALUE recv, ID mid)
{
    return rb_obj_respond_to(recv, mid, TRUE);
}
~~~



With the current behavior, the following in my opinion is incorrectly throwing the NameError in the method missing.

def method_missing(name, *args, &block)
     raise NameError, "#{name} not defined on #{self.inspect}"
end

puts "foo".gsub(/o/, "bar") 



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