[ruby-core:118244] [Ruby master Bug#20218] aset/masgn/op_asgn with keyword arguments

7 Jun 2024

      Issue #20218 has been updated by bughit (bug hit).

@matz You post argued against a strawman that preserving kwargs in `[]=` would necessarily violate kwarg separation. Since that is obviously not true, `[]=` can be given real, separated kwargs, instead of removing them entirely, the matter remains unsettled.
...
why is it preferable to remove the feature entirely from index assignment and have this incoherent inconsistency between [] (supports kwargs) and []= (doesn't support kwargs), rather than fix kwarg issues in []=?
----------------------------------------
Bug #20218: aset/masgn/op_asgn with keyword arguments
https://bugs.ruby-lang.org/issues/20218#change-108742

* Author: jeremyevans0 (Jeremy Evans)
* Status: Closed
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
I found that use of keyword arguments in multiple assignment is broken in 3.3 and master:

```ruby
h = {a: 1}
o = []
def o.[]=(*args, **kw)
  replace([args, kw])
end

# This segfaults as RHS argument is not a hash
o[1, a: 1], _ = [1, 2]

# This passes the RHS argument as keywords to the method, treating keyword splat as positional argument
o[1, **h], _ = [{b: 3}, 2]
o
# => [[1, {:a=>1}], {:b=>3}]
```

Before 3.3, keyword arguments were treated as positional arguments.

This is similar to #19918, but for keyword arguments instead of block arguments.

@matz indicated he wanted to prohibit block arguments in aset/masgn and presumably also op_asgn (making them SyntaxErrors).  Can we also prohibit keyword arguments in aset/masgn/op_asgn?

Note that aset treats keyword arguments as regular arguments:

```ruby
o[1, a: 1] = 2
o
# => [[1, {:a=>1}, 2], {}]

o[1, **h] = {b: 3}
o
# => [[1, {:a=>2}, {:b=>3}], {}]
```

While op_asgn treats keyword arguments as keywords:

```ruby
h = {a: 2}
o = []
def o.[](*args, **kw)
  concat([:[], args, kw])
  x = Object.new
  def x.+(v)
    [:x, v]
  end
  x
end
def o.[]=(*args, **kw)
  concat([:[]=, args, kw])
end
o[1, a: 1] += 2
o
# => [:[], [1], {:a=>1}, :[]=, [1, [:x, 2]], {:a=>1}]

o.clear
o[1, **h] += {b: 3}
o
# => [:[], [1], {:a=>2}, :[]=, [1, [:x, {:b=>3}]], {:a=>2}]
```

-- 
https://bugs.ruby-lang.org/