Bug #21139: Prism and parse.y parses `it = it` differently

• Status: Assigned
• Assignee: prism
• Target version: -
• ruby -v: ruby 3.5.0dev (2025-02-14T16:49:52Z master ee181d1bb7) +PRISM [x86_64-linux]
• Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[tompng (tomoya ishida)]

# ruby --parser=parse.y -e "42.tap { it = it; p it }"
nil
# ruby --parser=prism -e "42.tap { it = it; p it }"
42

[matz (Yukihiro Matsumoto)]

Assignment to it should be prohibited in the long run. Temporarily, I vote for Prism behavior.

Matz.

By the way, how can I clear assignee of the issue on Redmine?

[nobu (Nobuyoshi Nakada)]

matz (Yukihiro Matsumoto) wrote in #note-1:

Assignment to it should be prohibited in the long run. Temporarily, I vote for Prism behavior.

1. Currently, it is an ordinary local variable if assigned syntactically.
2. And a local variable is nil before assigned, even in its RHS, at the runtime.

The behavior in parse.y obeys above principles.

Do you want to change the rule 1, make it always special?

[nobu (Nobuyoshi Nakada)]

• Related to Bug #21137: Compound assignment operator with "it" parsed differently between parse.y and prism added

[nobu (Nobuyoshi Nakada)]

• Related to Bug #21138: The modifier expression with "it" is parsed differently in parse.y and Prism, which is unexpected in both. added

[alanwu (Alan Wu)]

• File clipboard-202503081702-idzz2.png clipboard-202503081702-idzz2.png added
• Status changed from Assigned to Open
• Assignee deleted (prism)

matz (Yukihiro Matsumoto) wrote in #note-1:

By the way, how can I clear assignee of the issue on Redmine?

The null option in the menu is the very first one and is rendered with a tiny height to make it hard to mouse over. It's easier to pick it with arrow keys.
Maybe the UX could be improved by tweaking some settings? CC @hsbt (Hiroshi SHIBATA)

[matz (Yukihiro Matsumoto)]

I said I prefer Prism behavior to parse.y. But It was due to my misunderstanding. it = it should initialize the local variable named it and the value should be nil, just like other x = x assignment in Ruby.
In the future, all assignment to it could be prohibited, maybe.

Matz.

[kddnewton (Kevin Newton)]

• Assignee set to prism

[hsbt (Hiroshi SHIBATA)]

• Status changed from Open to Assigned