Bug #12359: Named captures "conflict" warning is unnecessary and limits uses of named captures

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin14]
• Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN

[headius (Charles Nutter)]

There's currently a warning whenever a named capture in a regular expression has the same name as an already-declared local variable:

$ ruby23 -v -e 'x = 1; /(?<x>foo)/ =~ "foo"'
ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin14]
-e:1: warning: named capture conflicts a local variable - x

It also warns if you have two expressions using the same named capture:

$ ruby23 -v -e '/(?<x>foo)/ =~ "foo"; /(?<x>foo)/ =~ "foo"'
ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin14]
-e:1: warning: named capture conflicts a local variable - x

I do not believe either of these warnings are useful, since the only option for fixing them is to use a new variable name for every named capture.

I have a few more reasons, as well:

• Using the same named capture on both the "then" and "else" branches of an "if" statement will produce a warning. Changing one of the names requires additional work to join the to branches back together (since now there's no single variable resulting from them).
• It is not possible to initialize a variable with the same name as a named capture.
• The pattern that results from this warning is renaming a capture something like "name2" and then having "name = name2" after the match. Ugly.

Risks of removing the warning:

• When a named capture writes to an already-declared variable, that may surprise a user.

I think this risk is very low, especially since you'd have to turn on verbose mode to even see the warning.

[headius (Charles Nutter)]

See https://github.com/jruby/jruby/issues/3865 for a related issue in JRuby.

[headius (Charles Nutter)]

Related Ruby issue: https://bugs.ruby-lang.org/issues/9623

I think the warning needs to be softened to allow repeat use in literal regexp at the very least.

[headius (Charles Nutter)]

I have worked around this for now in JRuby by modifying the warning to only fire if a named capture was previously declared as something other than a named capture.

The commit to JRuby is here: https://github.com/jruby/jruby/commit/3ebc198860905084231d2a1356ea46718b86f2b0

It resolves the verbose noise issue we had with our date/format implementation.

I propose that the warning should only fire when it is a mixed-use variable, i.e. it is potentially assigned both as a named capture and as a normal local variable.

I'd actually like to get rid of it altogether, but it's really just this mixed-use case that needs a warning.

[Eregon (Benoit Daloze)]

• Related to Bug #9623: warning: "named capture conflicts a local variable" does not allow loops with named captures added

[nobu (Nobuyoshi Nakada)]

Agree.

https://github.com/ruby/ruby/compare/trunk...nobu:bug/12359-no-capture-warnings

[matz (Yukihiro Matsumoto)]

Accepted.

Matz.

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed

Applied in changeset r55396.

---

parse.y: no named capture conflict warnings

• parse.y (reg_named_capture_assign_iter): remove named capture
  conflict warnings. it is just annoying rather than useful.
  [ruby-core:75416] [Bug #12359]