Bug #20956: Nested alternation pattern matching bug in 3.4.0-rc1

• Status: Closed
• Assignee: prism
• Target version: -
• ruby -v: 3.4.0-rc1
• Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN

[nevans (Nicholas Evans)]

I think I've found a pattern matching bug in ruby 3.4.0-rc1.

[123, ["foo"]] in [_, /\Afoo\b/i | [/\Afoo\z/i, *]]

I also tested this in ruby 2.7.8, with:

case [123, ["foo"]]; in [_, /\Afoo\b/i | [/\Afoo\z/i, *]] then true; else false end

These return true in ruby 2.7.8, 3.0.7, 3.1.6, 3.2.6, and 3.3.6.
But they return false in 3.4.0-rc1.

I'm not sure if this is the best example, but it is a simplification of how I came across the bug.

Here are some other similar cases, which give the same result in 3.4.0-rc1 and 3.3.6:

irb(main):001> ["foo"] in /\Afoo\b/i | [/\Afoo\z/i, *]
=> true
irb(main):002> [123, ["foo"]] in [_, [/\Afoo\z/i, *]]
=> true
irb(main):003> [["foo"]] in [[/\Afoo\z/i, *]]
=> true

But this case exhibits the bug again:

irb(main):004> [["foo"]] in [/\Afoo\b/i | [/\Afoo\z/i, *]]
=> false

[nevans (Nicholas Evans)]

This would appear to be a bug in prism, or prism-adjacent code?

$ ruby -e 'pp ([["foo"]] in [/\Afoo\b/i | [/\Afoo\z/i, *]])'
false

$ ruby --parser=parse.y -e 'pp ([["foo"]] in [/\Afoo\b/i | [/\Afoo\z/i, *]])'
true

[alanwu (Alan Wu)]

• Assignee set to prism

[eightbitraptor (Matt V-H)]

This does appear to be an issue in prism_compile.c

Golfed down to

1 in [1 | [1]]

The instructions generated by Prism and parse.y are pretty significantly different, and appears to be related to nesting an Alternative Pattern inside an Array Pattern

generated instructions and diff output is in this gist

[eightbitraptor (Matt V-H)]

• Status changed from Open to Closed

Applied in changeset git|86cf18e01e8be1db6194b7cb27357150998d9056.

---

[PRISM] Recurse use_deconstructed_cache in Alternation Nodes

This fixes the behavioural difference between Prism and parse.y when
evaluating the following code

1 in [1 | [1]]

Fixes [Bug #20956]