Bug #22177: Prism rejects a nested destructuring target in a for loop index, while parse.y accepts it

• Status: Open
• Assignee: prism
• Target version: -
• ruby -v: ruby 4.0.5 (2026-05-20 revision 64336ffd0e) +PRISM [x86_64-linux]
• Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN

[shugo (Shugo Maeda)]

Summary

When the index of a for loop is a multiple-assignment target whose first element is itself a parenthesized target (for example (a, b), c), Prism reports a syntax error, but parse.y accepts it.
The same target works in a normal multiple assignment in both parsers, so the difference is specific to the for loop index.

Reproduction

for (a, b), c in [[[1, 2], 3]] do
  p [a, b, c]
end

parse.y (--parser=parse.y)

Accepted. Prints:

[1, 2, 3]

Prism (default, --parser=prism)

Rejected:

> 1 | for (a, b), c in [[[1, 2], 3]] do
    |     ^~~~~ unexpected write target

[shugo (Shugo Maeda)]

• Assignee set to prism

I've created a pull request: https://github.com/ruby/prism/pull/4156