Need specs for each_with_index + Enumerable method interaction

[headius]

There are at least three cases from jruby/jruby#8345 where an interaction between an each_with_index Enumerator and an Enumerable method was or is broken in JRuby:

• to_h
• uniq
• zip

Of these, we've fixed uniq and I have a fix in process for to_h but I'm not sure where to put specs for this kind of interaction.

My first thought was in Enumerable specs, since that's where I had to fix things, but I have not come up with a reproduction that doesn't use an each_with_index enumerator. All cases should be specified, though only a few exhibited this differing behavior between CRuby and JRuby.

Running the same example script against TruffleRuby shows similar issues, so I think getting these cases spec'ed would benefit all impls:

$ diff truffleruby_enum_args.txt ruby_enum_args.txt                  
1c1
< truffleruby 24.2.1, like ruby 3.3.7, Oracle GraalVM Native [arm64-darwin20]
---
> ruby 3.3.3 (2024-06-12 revision f1c7b6f435) [arm64-darwin23]
25c25
< filter_map [[1, 2], 0]
---
> filter_map [1, 2]
65c65
< to_h [[1, 2], 0]
---
> to_h [1, 2]
67c67
< uniq [[1, 2], 0]
---
> uniq [1, 2]
70c70
< zip [[[1, 2], 0]]
---
> zip [[1, 2], 0]

It's also possible CRuby is wrong to have special behavior for these methods.

[eregon]

Wrong first link I think?
I think it might be worth to discuss on https://bugs.ruby-lang.org/, though presumably the behavior of CRuby is already relied on and cannot really be changed, so it's also fine to spec the current behavior in parallel.
PR(s) welcome.