[ruby-core:114269] [Ruby master Bug#19759] Surprising automatic splat with ruby2_keywords_hash

Issue #19759 has been updated by nagachika (Tomoyuki Chikanaga).

Backport changed from 3.0: UNKNOWN, 3.1: REQUIRED, 3.2: REQUIRED to 3.0: UNKNOWN, 3.1: REQUIRED, 3.2: DONE

ruby_3_2 3354aacb73c65420a10cb41c0696e62dd1ba279b merged revision(s) 3874381c4483ba7794ac2abf157e265546f9bfa7.

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Bug #19759: Surprising automatic splat with ruby2_keywords_hash
https://bugs.ruby-lang.org/issues/19759#change-103960

* Author: alanwu (Alan Wu)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 3.2.2 (2023-03-30 revision e51014f9c0) [arm64-darwin22]
* Backport: 3.0: UNKNOWN, 3.1: REQUIRED, 3.2: DONE
----------------------------------------
On Ruby 3 and newer the following prints `[:aa, :bb, :k]`:

```ruby
def yielder(splat)
  yield([:a, :b], *splat)
end

yielder([[:aa, :bb], Hash.ruby2_keywords_hash({k: :k})]) do |a, b, k:|
  p [a, b, k]
end
```

It expanded the second array involved and `[:a, :b]` vanished.
This is surprising because automatic array expansion for blocks
used to only happen when passing one array in total.

The script prints `[[:a, :b], [:aa, :bb], :k]` with 2.7.3.

---

I found this while puzzling over this line in the autosplat logic:

```c
if (given_argc == (NIL_P(keyword_hash) ? 1 : 2) &&
```

It might be tempting to fix it by reversing part of commit:beae6cbf0fd and make it `if (given_argc == 1)`.
However that's incorrect due to inducing a behavior change for cases like
`yield(*[[:a, :b], keywords_hash])`.




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