Bug #22095: Prism rejects a multiple assignment in a rescue modifier value that parse.y accepts

• Status: Assigned
• Assignee: prism
• Target version: -
• ruby -v: ruby 4.0.0 (2025-12-25 revision 553f1675f3) +PRISM [arm64-darwin25]
• Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN

[Kai (kai matsudate)]

Problem

parse.y and Prism parse the following differently:

a rescue b, c = 1

parse.y accepts it, but Prism rejects it.

Cause

In the grammar, the value of a rescue modifier is a statement, so it may itself be a multiple assignment.

In Prism, multiple-assignment detection is only enabled at the statement level (PM_BINDING_POWER_STATEMENT). But Prism parses the rescue value with the rescue modifier's right binding power (PM_BINDING_POWER_COMPOSITION), so it does not recognize the multiple assignment and fails at the comma.

I opened a draft PR with a proposed fix: https://github.com/ruby/prism/pull/4128

[mame (Yusuke Endoh)]

• Status changed from Open to Assigned
• Assignee set to kddnewton (Kevin Newton)

[mame (Yusuke Endoh)]

• Assignee changed from kddnewton (Kevin Newton) to prism