[ruby-dev:51061] [Ruby master Bug#17925] Pattern matching syntax using semicolon one-line

[koic.ito@gmail.com]

Issue #17925 has been reported by koic (Koichi ITO).

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Bug #17925: Pattern matching syntax using semicolon one-line
https://bugs.ruby-lang.org/issues/17925

* Author: koic (Koichi ITO)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.0dev (2021-05-28T16:34:27Z master e56ba6231f) [x86_64-darwin19]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
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## Summary

There are the following differences between `case ... when` and` case ... in`.  Is this an expected behavior?

```console
% ruby -v
ruby 3.1.0dev (2021-05-28T16:34:27Z master e56ba6231f) [x86_64-darwin19]

% ruby -ce 'case expression when 42; end'
Syntax OK

% ruby -ce 'case expression in 42; end'
-e:1: warning: One-line pattern matching is experimental, and the behavior may change in future versions of Ruby!
-e:1: syntax error, unexpected `end', expecting `when'
case expression in 42; end
```

So, I have two concerns.

- Since the pattern matching syntax is different from `case ... when`,　can't user write semicolon one-line `case ... in` in the same semicolon one-line as `case ... when`?
- Does `case expression in 42; end` display an experimental warning of one-line pattern matching. Right?

This is reproduced in Ruby 3.1.0-dev and Ruby 3.0.1.

## Additional Information

NOTE 1: I understand that only syntax that doesn't use `case` and `end` is experimental one-line pattern matching syntax.

```
% ruby -ce 'expression in 42'
-e:1: warning: One-line pattern matching is experimental, and the behavior may change in future versions of Ruby!
Syntax OK
```

NOTE 2: The syntax is OK if a semicolon is used between `expression` and `in`. But `case ... when` is a valid syntax to omit.

```
% ruby -e ruby -ce 'case expression; in 42; end'
Syntax OK
```



-- 
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