Wrong number of arguments for Array#[]= subclass redefinition

[djberg96]

truffleruby 22.3.0, like ruby 3.0.3, GraalVM CE Native [x86_64-darwin]

Given the following code:

module Foo
  class Bar < Array
    def []=(index, obj)
      puts "ASET"
      super
    end

    def unshift(obj)
      puts "UNSHIFT"
      super
    end
  end
end

a = Foo::Bar.new
a[0] = 1
a.unshift(2)
p a

With Ruby I get the expected results:

ASET
UNSHIFT
[2, 1]

With TruffleRuby I get:

ASET
UNSHIFT
subclass_test.rb:3:in `[]=': wrong number of arguments (given 3, expected 2) (ArgumentError)
	from <internal:core> core/array.rb:1271:in `unshift'
	from subclass_test.rb:10:in `unshift'
	from subclass_test.rb:17:in `<main>'

[eregon]

Thanks for the report, we'll take a look.
Note that that Array subclass is incompatible, Array#[]= supports 3 arguments, e.g. ary[0, 1] = value.
In this case it's due to unshift being implemented as self[0, 0] = values.

Is there real code depending on this?

[eregon]

Is there real code depending on this?

Ah it's probably from https://github.com/djberg96/berger_spec I guess.
I think we should fix this, but the priority is low if it doesn't happen in gems/apps, also due to only happening with a slightly-broken Array subclass.

[djberg96]

Is there real code depending on this?

In this case it was actually https://github.com/djberg96/html-table :)

[djberg96]

Note that that Array subclass is incompatible, Array#[]= supports 3 arguments, e.g. ary[0, 1] = value.
In this case it's due to unshift being implemented as self[0, 0] = values.

Ah, that might actually be a bug in my code then that happened to smoke out something.

[eregon]

I see, https://github.com/djberg96/html-table/blob/902b12f72a3f7130331356cbd4b4de9bd7840e15/lib/html/table.rb#L207
In general it's best to not subclass core classes but for compatibility TruffleRuby should support this case.

It's a bit unfortunate because to fix that we probably have to make the core library code less clear & efficient (e.g., using bind_call to call the original []=), or increase maintenance by having unshift written in Java and duplicating logic.

[djberg96]

Is it something that could be deferred for subclasses?

[eregon]

Not really, that would probably be more complicated. Anyway I think we need to take that cost here and choose the better alternative of the two.

[andrykonchin]

Fixed in 534659f.