Double-splat operator destructively modifies hash – is this a Ruby bug? -

i noticed find surprising behavior ** (double-splat) operator in ruby 2.1.1.

when key-value pairs used before **hash, hash remains unmodified; however, when key-value pairs used after **hash, hash permanently modified.

h = { b: 2 }  { a: 1, **h }        # => { a: 1, b: 2 } h                    # => { b: 2 }  { a: 1, **h, c: 3 }  # => { a: 1, b: 2, c: 3 } h                    # => { b: 2 }  { **h, c: 3 }        # => { b: 2, c: 3 } h                    # => { b: 2, c: 3 } 

for comparison, consider behavior of single-* operator on arrays:

a = [2]  [1, *a]     # => [1, 2]           # => [2]  [1, *a, 3]  # => [1, 2, 3]           # => [2]  [*a, 3]     # => [2, 3]           # => [2] 

the array remains unchanged throughout.

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do suppose sometimes-destructive behavior of ** intentional, or more bug?

in either case, documentation describing how ** operator meant work?

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i asked question in ruby forum.

update

the bug fixed in ruby 2.1.3+.

the answers question seem be:

1. it's bug, rather intentional.

2. the behavior of ** operator documented briefly in core library rdoc.

thanks suggestions of several commenters, i've posted bug ruby trunk issue tracker.

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update:

the bug fixed in changeset r45724. comment there "keyword splat should non-destructive," makes authoritative answer.