[ruby-core:106626] [Ruby master Bug#18405] Regression in Struct member setter method parameters

Issue #18405 has been updated by ko1 (Koichi Sasada).


https://github.com/ruby/ruby/pull/5252

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Bug #18405: Regression in Struct member setter method parameters
https://bugs.ruby-lang.org/issues/18405#change-95296

* Author: ufuk (Ufuk Kayserilioglu)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.0dev (2021-12-08T10:11:02Z master 3021c3cedc) [x86_64-darwin21]
* Backport: 2.6: DONTNEED, 2.7: DONTNEED, 3.0: DONTNEED
----------------------------------------
Since the [PR optimizing Struct accessors](https://github.com/ruby/ruby/pull/5131) was merged, the behaviour of `Method#parameters` on Struct member setter methods has changed.

**Before:**
```ruby
S = Struct.new(:foo)
S.instance_method(:foo=).parameters
#=> [[:req, :_]]
```

**After:**
```ruby
S = Struct.new(:foo)
S.instance_method(:foo=).parameters
#=> [[:rest]]
```

This is probably a side-effect of the new optimized accessor implementation that was missed in review. 

If this change in behaviour was intentional, though, I think it should be fixed, since it is a little strange for a setter method to claim that it takes zero or more parameters.



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