[ruby-core:106632] [Ruby master Bug#18396] An unexpected "hash value omission" syntax error when without parentheses call expr follows

Issue #18396 has been updated by zverok (Victor Shepelev).


The current state of things is indeed quite confusing (Wrote a small blog post on it: https://zverok.github.io/blog/2021-12-08-value-omission-debug.html).

As far as I can understand, the kind of old code that would be affected should look like this:
```ruby
method_call foo:
  bar
```

While using parenthesis in non-DSL method calls is the most common style currently, it is not the only one.
For example, Seattle.rb's style (minitest) have optional parenthesis

Also, I suspect there could be a frequently/sometimes used style at least when using Rails DSL:
```ruby
validate :something, if: -> {condition }

# ...when `condition` grows longer, I suspect _some_ codebases might do this:
validate :something, if: 
                     -> {condition }
```

Another parenthesis-less DSLs are assertions (of the same minitest and other "testunit"-alike frameworks).

The worst thing here, I can't think of a way of catching this incompatibility automatically, just after updating on 3.2 _some_ code might break, and in a quite subtle way (for example, DSL methods receiving something _accidentally available_ in current scope), or in a baffling one like ` undefined local variable or method 'if'` — ugh what.

As an alternative, I was thinking maybe prohibit methods with keyword argument omission **and** parenthesis omission? E.g. make this:

```ruby
p x: 1, y: 1  # valid
p x:, y: 1    # valid
p x: 1, y:    # invalid, ambiguity!
p(x: 1, y:)   # valid
```


----------------------------------------
Bug #18396: An unexpected "hash value omission" syntax error when without parentheses call expr follows
https://bugs.ruby-lang.org/issues/18396#change-95305

* Author: koic (Koichi ITO)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.0dev (2021-12-07T23:18:11Z master 4a3e7984bf) [x86_64-darwin19]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
## Summary

I encountered an unexpected "hash value omission" syntax error when parentheses call expression follows:

```ruby
% cat example.rb
foo key:
foo arg
```

```console
% ruby -cv /tmp/b.rb
ruby 3.1.0dev (2021-12-07T23:18:11Z master 4a3e7984bf) [x86_64-darwin19]
example.rb:2: syntax error, unexpected local variable or method, expecting `do' or '{' or '('
foo arg
```

## Additional Information

The following is a traditional usage.

```ruby
# No errors.
foo key: key
foo arg
```

A syntax error is unexpectedly raised when hash value omission argument without parentheses is followed by a method call without parentheses.

```ruby
# No errors is expected, but syntax error is raised.
foo key:
foo arg
```

No error occurs if any of the calls have parentheses.

```ruby
# No errors.
foo(key:)
foo arg
```

```ruby
# Also no errors.
foo key:
foo(arg)
```

No error occurs when calling alone.

```ruby
# No errors.
foo key:
```

I encountered this error while trying to apply hash value omission to  RSpec code of a real-world application (proprietary) .
But this is a new Ruby 3.1 syntax and may not be supported yet. Thank you.



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