[ruby-core:110816] [Ruby master Bug#19113] Inconsistency in retention of compare_by_identity flag in Hash methods

["jeremyevans0 (Jeremy Evans)" <noreply@ruby-lang.org>]

Issue #19113 has been updated by jeremyevans0 (Jeremy Evans).


headius (Charles Nutter) wrote in #note-6:
> Even though it's a "new Hash", it is supposed to be populated with "the given objects". If losing identity comparison means some keys don't get populated, I would consider that broken.

If you turn the hash into an array, you have the same `given objects`, but different results:

```ruby
h = {'a'=>1}.compare_by_identity
h['a'] = 2
Hash[h]      # {'a'=>1, 'a'=>2}.compare_by_identity
Hash[h.to_a] # {'a'=>2}
```

Not respecting the compare_by_identity flag (as my PR does) would make `Hash[h] == Hash[h.to_a]`.

I would say that `populated with the given objects` does not imply `resulting in the same set of keys.`  Also, `given objects` (not `given object`) implies it looks at the content of the hash, but not the hash object itself (hence why default value/proc is not copied).

Here's the pseudocode I would consider based on the documentation:

```ruby
# new hash
output_hash = {}
# populated with the given objects
input_hash_or_array.each do |k, v|
  output_hash[k] = v
end
output_hash
```

There are certainly valid arguments for both sides, so we'll have to see what @matz decides. 

----------------------------------------
Bug #19113: Inconsistency in retention of compare_by_identity flag in Hash methods
https://bugs.ruby-lang.org/issues/19113#change-100174

* Author: jeremyevans0 (Jeremy Evans)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.2.0dev (2022-11-07T17:29:28Z master 9001e53e68) [x86_64-openbsd7.2]
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
`Hash.[]` and `Hash.ruby2_keywords_hash` retain the compare_by_identity flag for non-empty hashes, but do not retain it for empty hashes:

```ruby
hs = [{}.compare_by_identity, {:a=>1}.compare_by_identity]
hs.map{|h| Hash[h].compare_by_identity?}
# => [false, true]
hs.map{|h| Hash.ruby2_keywords_hash(h).compare_by_identity?}
# => [false, true]
```

This inconsistency seems like a bug.

`Hash#compact` always drops the compare_by_identity flag, but it is documented as returning a copy of self, implying the compare_by_identity flag is kept (since #dup and #clone retain the flag).

```ruby
{}.compare_by_identity.compact.compare_by_identity?
# => false
```

I'm not sure whether is a bug, because it is consistent, but I think retaining the flag makes more sense.

I'll try to work on a fix for both of these issues tomorrow.



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