From: "yuki300 (Yuki Hiro) via ruby-core" <ruby-core@...>
Date: 2026-09-14T19:02:53+00:00
Subject: [ruby-core:126699] [Ruby Bug#22314] parse.y does not check **rest bindings in patterns (duplicate / alternation)

Issue #22314 has been reported by yuki300 (Yuki Hiro).

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Bug #22314: parse.y does not check **rest bindings in patterns (duplicate / alternation)
https://bugs.ruby-lang.org/issues/22314

* Author: yuki300 (Yuki Hiro)
* Status: Open
* ruby -v: ruby 4.0.4 (2026-05-12 revision b89eb1bcbf) +PRISM [arm64-darwin25]
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
`p_kwrest` is the only binding rule in patterns that does not go through
`error_duplicate_pattern_variable`. `p_variable`, `p_rest` and the `{a:}`
shorthand in `p_kw` all call it before `assignable`; for `**rest` the name is
handed straight to `assignable` in `new_hash_pattern_tail`. So `**rest` is
never registered in `pvtbl`, never checked against `in_alt_pattern`, and never
sets `capture_in_pattern`.

## Duplicate binding

    case { a: 1, b: 2 }
    in { a: x, **x } then p x
    end

    $ ruby --parser=parse.y dup.rb
    {b: 2}                          # accepted and runs; x is the rest hash
    $ ruby --parser=prism dup.rb
    dup.rb:2: duplicated variable name (SyntaxError)

`in a, a` and `in a, {a:}` are rejected by parse.y as expected; only `**rest`
slips through. This is not a 4.0 regression: parse.y in 3.4.9 also accepts it,
and Prism 3.4.9 already rejects it.

## Binding inside an alternative pattern

    case 1
    in 1 | { a: 1, **rest }
    end

    $ ruby --parser=parse.y -c alt.rb
    Syntax OK
    $ ruby --parser=prism -c alt.rb
    alt.rb:2: variable capture in alternative pattern (SyntaxError)

    $ ruby --parser=parse.y alt.rb      # compile.c still catches it at run time
    alt.rb:2: illegal variable in alternative pattern (rest)

`*rest` in the same position is rejected at parse time by both parsers. This
form only diverges since 4.0, where https://bugs.ruby-lang.org/issues/21572
moved the check to parse time; in 3.4.9 both parsers accept it under `-c` and
reject it at compile time.

## Fix

Call `error_duplicate_pattern_variable` in `p_kwrest`, as `p_rest` does.
One line plus tests: https://github.com/ruby/ruby/pull/18831

It should land after the fix for https://bugs.ruby-lang.org/issues/22310
(https://github.com/ruby/ruby/pull/18830): on its own, making `**rest` set
`capture_in_pattern` would newly reject an unrelated sibling alternation such
as `in [ { **rest }, 1 | 2 ]` under the current `p_alt` check. With that fix
applied first, both parsers agree on every form I tried. Since this turns code
parse.y has accepted at least since 3.4 into a SyntaxError, I am not
requesting a backport.

## Environment

    ruby 4.0.4 (2026-05-12 revision b89eb1bcbf) +PRISM [arm64-darwin25]   # rejects with --parser=prism only
    ruby 3.4.9 (2026-03-11 revision 76cca827ab) +PRISM [arm64-darwin25]   # duplicate case: same divergence



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