Bug #1125: [*x] (array consisting only of a splat) does not necessarily return a new array
http://redmine.ruby-lang.org/issues/show/1125

Author: Daniel Luz
Status: Open, Priority: Normal
ruby -v: ruby 1.8.7 (2008-08-11 patchlevel 72) [x86_64-linux]

For [*x], these are basically the possible outcomes:

1. if x is an Array, returns it unmodified.
2. elsif x responds to to_ary (to_a on 1.9.1), invokes that method and returns its result unmodified.
3. else, returns a new array with x as its only element.

Cases #1 and #2 IMO violate the POLS, as I expected that an array literal would always return a new array. The practical consequence here is that I expected I'd be free to modify it without side effects. (For comparison, "#{x}" always returns a new string)

Simple test case:
  x = [1, 2, 3]
  [*x] << 4
  p x # => [1, 2, 3, 4]


Thus, I propose ensuring these two cases always return new arrays.
One possible solution would be dup'ing the resulting array (I guess that'd have a rather low cost; the third case would result in an unnecessary dup, but at least it's just a single-item array). Another one would be to dumb down the interpreter, making it create a zero-length array and then concat the result of the splat to it.


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