Issue #13558 has been reported by kke (Kimmo Lehto).

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Bug #13558: Array#insert does not typecheck the position argument if it's the only argument
https://bugs.ruby-lang.org/issues/13558

* Author: kke (Kimmo Lehto)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-darwin16]
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
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**rb_ary_insert** does this:

~~~ c
if (argc == 1) return ary;
~~~


For Ruby this means:

~~~ ruby
> [].insert("foo") 
 => []
> [].insert(Exception.new)
 => []
~~~

I would expect this to raise a TypeError. The problem here is that if you forget the position argument, because perhaps you thought Array#insert would be an alias for Array#unshift, you get unexpected results and no errors.

If you try to insert two items it fails nicely:

~~~ ruby
> [].insert('foo', 'foo')
TypeError: no implicit conversion of String into Integer
~~~





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