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Only report real captures inside alternation patterns - #4236
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When a variable is captured anywhere in a pattern, the operands of each
alternation are visited to report captures inside them. That visitor
treated every local variable target as a capture, including
underscore-prefixed names and targets that belong to a lambda, a string
interpolation, or a pinned expression:
case foo
in [bar, 1 | _baz] # error
in [bar, "#{case 1; in baz; end}" | 2] # error
end
Without `bar` both are accepted. Skip underscore-prefixed names, like
parse_pattern_capture does, and only descend into the nodes that make up
the pattern itself. Nested alternations are skipped too, their operands
were already checked when they were parsed. Before this, a capture like
`1 => [x, (1 | y) | 2]` was reported twice.
The new fixture is excluded from the tests that compare against parse.y,
which currently rejects these forms because of the capture of `bar`:
https://bugs.ruby-lang.org/issues/22310
kddnewton
approved these changes
Sep 28, 2026
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When a variable is captured anywhere in a pattern, Prism also reports underscore-prefixed names and local variable targets inside lambdas, string interpolation and pinned expressions as captures in an alternation:
Both are accepted without
bar. This skips underscore-prefixed names likeparse_pattern_capturedoes and only visits the nodes that make up the pattern. Nested alternations are skipped as well since their operands were already checked when they were parsed. That also gets rid of the double error forin [x, (1 | y) | 2].The new fixture is excluded from the parse.y comparisons. parse.y currently rejects these forms for a different reason (https://bugs.ruby-lang.org/issues/22310, fix proposed in ruby/ruby#18830) and the exclusions can go once that lands.
Fixes #4235