[ruby-core:70165] [Ruby trunk - Bug #10969] public_send in combination with method_missing raises NameError instead of NoMethodError

Issue #10969 has been updated by Nobuyoshi Nakada.

Description updated
Backport set to 2.0.0: DONTNEED, 2.1: DONTNEED, 2.2: REQUIRED

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Bug #10969: public_send in combination with method_missing raises NameError instead of NoMethodError
https://bugs.ruby-lang.org/issues/10969#change-53593

* Author: Yves Senn
* Status: Closed
* Priority: Normal
* Assignee: 
* ruby -v: ruby 2.2.1p85 (2015-02-26 revision 49769) [x86_64-darwin14]
* Backport: 2.0.0: DONTNEED, 2.1: DONTNEED, 2.2: REQUIRED
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While working on the Rails project, specifically this issue https://github.com/rails/rails/issues/19297 I discovered that `public_send` can raise a `NameError` instead of a `NoMethodError`.

Following is a minimal reproduction scenario to trigger the bug. A more detailed example can be found in this Gist: https://gist.github.com/senny/9864a138defa322ed807

~~~ruby
class Person
  def implicit_assignment
    nope rescue nil
    public_send "nope="
  end

  def method_missing(*args)
    super
  end
end

a = Person.new
a.implicit_assignment
# test.rb:13:in `method_missing': undefined local variable or method `nope=' for #<Person:0x007f91d1052ef8> (NameError)
# 	from test.rb:4:in `public_send'
# 	from test.rb:4:in `implicit_assignment'
# 	from test.rb:24:in `<main>'
~~~

### What a found out during debugging:

I am not a C programmer and have very little experience in that field. While debugging the issue I could make some observations what's going on.

The error is being raised in `raise_method_missing` (https://github.com/ruby/ruby/blob/7790f37efdd8dd42a0a43c3206f6afdd43f8e86a/vm_eval.c#L704-L706):

~~~c
else if (last_call_status & NOEX_VCALL) {
  format = "undefined local variable or method `%s' for %s";
  exc = rb_eNameError;
~~~

`last_call_status` is stored on the current thread (https://github.com/ruby/ruby/blob/7790f37efdd8dd42a0a43c3206f6afdd43f8e86a/vm_eval.c#L655):

The thread struct is modified in `vm_call_method_missing` (https://github.com/ruby/ruby/blob/7790f37efdd8dd42a0a43c3206f6afdd43f8e86a/vm_insnhelper.c#L1668):

~~~c
th->method_missing_reason = ci->aux.missing_reason;
~~~

Now the problem is, that the call to `public_send` with the method name containing an `=` sign, does not modify the thread struct. This means that it still contains the value assigned from the previous call. That's what `nope rescue nil` in the reproduction is used for. It assigns `NOEX_VCALL` to that struct.



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