[ruby-core:102147] [Ruby master Feature#16806] Struct#initialize accepts keyword arguments too by default

Issue #16806 has been updated by k0kubun (Takashi Kokubun).

Assignee set to k0kubun (Takashi Kokubun)
Status changed from Closed to Assigned

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Feature #16806: Struct#initialize accepts keyword arguments too by default
https://bugs.ruby-lang.org/issues/16806#change-89999

* Author: k0kubun (Takashi Kokubun)
* Status: Assigned
* Priority: Normal
* Assignee: k0kubun (Takashi Kokubun)
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## Proposal

```rb
Post = Struct.new(:id, :name)

# In addition to this,
Post.new(1, "hello") #=> #<struct Post id=1, name="hello">

# Let the following initialization also work
Post.new(id: 1, name: "hello") #=> #<struct Post id=1, name="hello">
```

### Known incompatibility

* `Post.new(id: 1, name: "hello")` will be `#<struct Post id=1, name="hello">` instead of `#<struct Post id={:id=>1, :name=>"hello"}, name=nil>`
  * Struct initialization only using keyword arguments should be warned in Ruby 3.0. **This feature should be introduced in Ruby 3.1 or later.**

### Edge cases

* When keyword arguments and positional arguments are mixed: `Post.new(1, name: "hello")`
  * This should continue to work like Ruby 2: `#<struct Post id=1, name={:name="hello"}>`
* Only keywords are given but they include an invalid member: `Post.new(foo: "bar")`
  * ArgumentError (unknown keywords: foo)
* When `keyword_init` is used
  * nil: default behavior. Positional arguments given use positional init. Keyword arguments without positional arguments treated as positional in 3.0 with warning, and treated as keyword init in Ruby 3.1.
  * true: Require keyword init, disallow positional init.
  * false: Treat keywords as positional hash.

## Use cases

* Simplify a struct definition where [Feature #11925] is used.
  * When we introduced [Feature #11925], @mame thought we don't need `keyword_init: true` once keyword args are separated (https://docs.google.com/document/d/1XbUbch8_eTqh21FOwj9a_X-ZyJyCBjxkq8rWwfpf5BM/edit#). That's what this ticket is all about.
     * However, the keyword arguments separation was done differently from what we expected at the moment. So we need to deal with the "Known incompatibility".
  * Matz objected to having a new keyword argument (`immutable: true`) in `Struct.new` at https://bugs.ruby-lang.org/issues/16769#note-8. So `keyword_init: true` seems also against Ruby's design. Now we should be able to skip specifying the option for consistency in the language design.



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