Bug #16797
closed
Array#flatten not checking `respond_to? :to_ary`
Description
def (a = Object.new).method_missing(...)
Object.new
end
[a].flatten # TypeError
It should check whether an object responds to to_ary before trying to convert it into an array.
Updated by nobu (Nobuyoshi Nakada) about 4 years ago
- Status changed from Open to Rejected
Read the error message.
-:4:in `flatten': can't convert Object to Array (Object#to_ary gives Object) (TypeError)
Updated by Dan0042 (Daniel DeLorme) almost 4 years ago
Actually what happens here is pretty interesting. This is all done via rb_check_array_type which attempts to convert an object by using to_ary.
If your object defines its own respond_to? method, it is used to determine if to_ary exists and should be called.
a = Object.new
def a.method_missing(...); Object.new; end
def a.respond_to?(...); super; end
[a].flatten #=> [#<Object:0x000055ca978ce058>]
b = Object.new
def b.to_ary; bug!; end
def b.respond_to?(...); false; end
[b].flatten #=> [#<Object:0x000055ca9789eab0>]
Otherwise if the object only has the default respond_to? method, its tries to call method_missing. NoMethodError is rescued and taken to mean that the object does not respond to to_ary, and the object is not flattened.
Without a NoMethodError it means the object effectively responds to to_ary, and the return type must be Array.
a = Object.new
def a.method_missing(...); 42; end
[a].flatten #=> TypeError (can't convert Object to Array (Object#to_ary gives Integer))
def a.method_missing(...); [[42]]; end
[a].flatten #=> [42]
def a.method_missing(...); super; end #NoMethodError triggered via super
[a].flatten #=> [#<Object:0x00005603341f3b18>]
def a.method_missing(...); 42.foobar; end #NoMethodError is actually because of foobar
[a].flatten #=> [#<Object:0x0000560334024300>]
Updated by UlyssesZhan (Ulysses Zhan) almost 4 years ago
nobu (Nobuyoshi Nakada) wrote in #note-1:
Read the error message.
-:4:in `flatten': can't convert Object to Array (Object#to_ary gives Object) (TypeError)
Read what I wrote:
It should check whether an object responds to to_ary before trying to convert it into an array.
I know it used to_ary method and got an unexpected result, but in fact calling a.to_ary itself is unexpected because a.respond_to? :to_ary is false.
Detecting NoMethodError is a bit silly.