Bug #22140: Prism rejects a trailing comma in a pattern-matching loop condition before do, while parse.y accepts it

• Status: Open
• Assignee: prism
• Target version: -
• ruby -v: ruby 4.1.0dev (2026-07-02T00:24:43Z master e91f477e21) +PRISM [arm64-darwin25]
• Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN

[Kai (kai matsudate)]

Prism rejects the following code, while parse.y accepts it:

Prism:

$ ruby --parser=prism -ce 'while a in b, do end'
ruby: -e:1: syntax error found (SyntaxError)
> 1 | while a in b, do end
    |               ^~ expected a pattern expression after `,`

parse.y:

$ ruby --parser=parse.y -ce 'while a in b, do end'
Syntax OK

The same appears with until and for loops.

I have a draft patch here: https://github.com/ruby/prism/pull/4154

[Earlopain (Earlopain _)]

• Assignee set to prism