[ruby-core:96970] [Ruby master Bug#16519] pp [Hash.ruby2_keywords_hash({})] shows `[nil]`

Issue #16519 has been updated by Eregon (Benoit Daloze).


> To paraphrase @Eregon , if a ruby committer heavily involved in the process cannot figure out even why a problem occurs, let alone the solution, what chance does a standard ruby programmer have?

For the record, by "I don't know why it goes wrong." I meant "This is after midnight and I have no time to investigate it further today".
But yes, I'm surprised a ruby2_keywords flagged Hash is just removed and "lost".
I think we might want to fix that.

I think the actual issue here might be that we remove empty keyword arguments, which is also inconsistent with non-empty ones when they are given to a method not accepting keyword arguments:
```ruby
def target(*args)
  args
end

# target does not accept keyword arguments, it shouldn't matter if we pass a last positional Hash or use keyword arguments syntax.
# But actually it does in 2.7.0 and master!
h = {}
p target(**{a: 1}) # => [{:a=>1}]
p target(**h)     # => [] in 2.7.0 and master (inconsistent with above), [{}] in 2.6 and ruby2_keywords by default
p target(h)       # => [{}]
```

We need to remove empty keyword arguments for one reason: to make `*args, **kwargs` delegation work with a target method not taking kwargs:
```ruby
def target(a, b = nil)
  [a, b]
end

def delegate(*args, **kwargs)
  target(*args, **kwargs)
end

p delegate(1) # => [1, {}] in 2.6.5, [1, nil] in 2.7.0, master and ruby2_keywords by default
```

Note that if `target` does accept keyword arguments, it would not change anything whether we pass or not an empty keyword Hash in Ruby 3.0+.

Here are two ideas to make `*args, **kwargs` delegation work, but not remove empty keyword hashes when passed to a method not accepting keyword arguments:
* When the keyword Hash is created out of nothing, i.e., by `def m(**hash_created_by_double_splat)` and `m` was not passed any keyword argument, we could remember that, and when calling another method with `foo(**hash_created_by_double_splat)` remove that Hash, since the user never intended it. For other cases, keep pass the keyword Hash just like for non-empty keyword hashes, since the user passed keywords explicitly.
* Maybe ruby2_keywords-flagged Hash should never be removed. I need to think more about that case and experiment with it.

@jeremyevans0 @mame What do you think of that?

----------------------------------------
Bug #16519: pp [Hash.ruby2_keywords_hash({})] shows `[nil]`
https://bugs.ruby-lang.org/issues/16519#change-83994

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.8.0dev (2020-01-21T13:45:10Z master 5798d35ff6) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: REQUIRED
----------------------------------------
This happens on `master`:

```
$ ruby -ve 'ruby2_keywords def flag(*a); a.last; end; pp [flag(**{})]'
ruby 2.8.0dev (2020-01-21T13:45:10Z master 5798d35ff6) [x86_64-linux]
[nil]
```

Of course it should be `[{}]`, as it is for `pp [{}]`.

On 2.7.0 it warns (should be fixed, it's valid to `pp` a flagged Hash):
```
$ ruby -ve 'ruby2_keywords def flag(*a); a.last; end; pp [flag(**{})]'
ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-linux]
[/home/eregon/.rubies/ruby-2.7.0/lib/ruby/2.7.0/pp.rb:226: warning: Passing the keyword argument as the last hash parameter is deprecated
/home/eregon/.rubies/ruby-2.7.0/lib/ruby/2.7.0/pp.rb:334: warning: The called method is defined here
{}]
```

The warning being in the middle of the output is a fun fact here.
Lines it refers to (still the same on current master):
https://github.com/ruby/ruby/blob/v2_7_0/lib/pp.rb#L226
https://github.com/ruby/ruby/blob/v2_7_0/lib/pp.rb#L334

This is very confusing as it can happen during `test-all` and then show output such as:
```
<[{:a=>1}]> expected but was
<[{:a=>1}, nil]>.
```
when the reality is (can be verified with `p` before the `assert_equal`):
```
<[{:a=>1}]> expected but was
<[{:a=>1}, {}]>.
```



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