Issue #16182 has been reported by mame (Yusuke Endoh).

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Feature #16182: Should `expr in a, b, c` be allowed or not?
https://bugs.ruby-lang.org/issues/16182

* Author: mame (Yusuke Endoh)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
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In #15865, a new syntax `<expr> in <pattern>` was introduced.  By using this, we can write:

```
json = { foo: 1, bar: 2}

if json in { foo:, bar: }
  p [foo, bar] #=> [1, 2]
end
```

However, we cannot write:

```
p(json in { foo:, bar: }) #=> expected: true, actual: syntax error
```

This is because `<expr> in <pattern>` is an expression but not an argument.  For example, `foo(json in a, b, c)` is ambiguous: it is considered `foo((json in a), b, c)` and `foo((json in a, b, c))`.

What should we do?

1. Do nothing; we admit that it is a spec
2. Revert the feature
3. Disallow a pattern like `a, b, c` or `a:, b:, c:` in this one-line pattern matching syntax; we ask a user to write `json in [a, b, c]` or `json in {a:, b:, c:}`



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