[ruby-core:111113] [Ruby master Feature#19107] Allow trailing comma in method signature

Issue #19107 has been updated by matz (Yukihiro Matsumoto).


I don't care for consistency here (since formal arguments and actual arguments are different).
I am not sure for convenience. Compare to actual arguments, there's less chance to rewrite/update formal arguments.
Is there an actual case where this proposal is convenient?

Matz.


----------------------------------------
Feature #19107: Allow trailing comma in method signature
https://bugs.ruby-lang.org/issues/19107#change-100385

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
A popular style for multiline arrays, hashes or method calls, is to use trailing commas:

```ruby
array = [
  1,
  2,
  3,
]

hash = {
  foo: 1,
  bar: 2,
  baz: 3,
} 

Some.method(
  1,
  2,
  foo: 3,
)
```

The main reason to do this is to avoid unnecessary noise when adding one extra element:

```diff
diff --git a/foo.rb b/foo.rb
index b2689a7e4f..ddb7dc3552 100644
--- a/foo.rb
+++ b/foo.rb
@@ -1,4 +1,5 @@
 Foo.bar(
   foo: 1,
-  bar: 2
+  bar: 2,
+  baz: 3
 )
```

However, this pattern doesn't work with method declarations:

```ruby
def foo(bar:,) # syntax error, unexpected ')'
``` 

### Proposal

For consistency and convenience I propose to allow trailing commas in method declarations.



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