Add a spec for Enumerator#size of zero

[edgibbs]

👋🏾 Found this when doing some work on garnetjs. After doing some digging verified it was a potential issue and impacted Opal in the same way. Enumerator#size returns the size of the enumerator, or nil when the size cannot be determined without iterating. 'core/enumerator/size_spec.rb' pins an Integer size (100), nil, and a Proc, but never zero.

The case cannot fail on CRuby, where 0 is truthy and INT2FIX(0) is a nonzero VALUE distinct from Qnil. It can fail on implementations hosted in a language where 0 is falsy: Opal returns nil for Enumerator.new(0) {}.size, because opal/corelib/enumerator.rb assigns the size with arguments[0] || nil and JS || collapses a size of 0 to nil. The neighbouring Enumerator.new(100) example passes there, so nothing in the suite currently detects this.

Matchers follow the existing examples in the file.

Reproduction steps

Reproducing the Opal failure (requires Node, since Opal compiles to JS):

$ gem install opal          # tested with 1.8.3

$ opal --no-exit -e 'p Enumerator.new(100) {}.size; p Enumerator.new(0) {}.size' 
100
nil

$ ruby --disable-gems -e 'p Enumerator.new(100) {}.size; p Enumerator.new(0) {}.size'
100
0

The existing Enumerator.new(100) example passes on Opal. Only the new case will fail.

Cause — opal/corelib/enumerator.rb:33, inside Enumerator#initialize:

@size = `arguments[0] || nil`

That's inline JavaScript. 0 || nil evaluates to nil because 0 is falsy in JS, so a known size of zero becomes "size unknown." Nothing downstream can recover it — #size (line 56) just returns @size:

def size
  @size.respond_to?(:call) ? @size.call(*@args) : @size
end