Issue #12568 has been reported by Yui NARUSE.

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Bug #12568: wrong ArgumentError if an array is given for instance_exec with optional argument
https://bugs.ruby-lang.org/issues/12568

* Author: Yui NARUSE
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: ruby 2.4.0dev (2016-07-08 trunk 55607) [x86_64-darwin15]
* Backport: 2.1: DONTNEED, 2.2: REQUIRED, 2.3: REQUIRED
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From Ruby 2.2 to trunk, it wrongly raise ArgumentError as follows:

Sample code:
```ruby
instance_exec([1,2,3]){|a| p a}
instance_exec([1,2,3]){|a=[]| p a}
instance_exec([1,2,3], &->(a){ p a })
->(a=[]){ p a }.to_proc.call([1,2,3])
instance_exec([1,2,3], &->(a=[]){ p a })
```

Expected result:
```
[1, 2, 3]
[1, 2, 3]
[1, 2, 3]
[1, 2, 3]
[1, 2, 3]
```

Actual result:
```
[1, 2, 3]
[1, 2, 3]
[1, 2, 3]
[1, 2, 3]
test.rb:7:in `block in <main>': wrong number of arguments (given 3, expected 0..1) (ArgumentError)
	from test.rb:7:in `instance_exec'
	from test.rb:7:in `<main>'
```



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