Style/CaseLikeIf unsafe if conditionally executed code needs MatchData

[JoeCohen]

Style/CaseLikeIf is unsafe in edge cases. It gives false positives if both:

• if or elsif includes a regular expression, and
• the code that's executed when the condition is true depends on something in the match other than the existence or numerical position of a match.

---

Expected behavior

I expect RuboCop to not generate a Style/CaseLikeIf offense.

Actual behavior

RuboCop generates a Style/CaseLikeIf offense.

Steps to reproduce the problem

Create an if-elsif where the if or elsif includes =~ or .match and the conditionally executed code depends on something in the match other than true/false or the position of the match.

In my case the problematic code was

      if /^(?<yr>\d{4})$/ =~ val
        yyyymmdd([yr, 1, 1], [yr, 12, 31])
      elsif /^(?<yr>\d{4})-(?<mo>\d\d?)$/ =~ val
        yyyymmdd([yr, mo, 1], [yr, mo, 31])
      elsif /^(?<yr>\d{4})-(?<mo>\d\d?)-(?<day>\d\d?)$/ =~ val
        yyyymmdd([yr, mo, day], [yr, mo, day])
      elsif /^(?<yr1>\d{4})-(?<yr2>\d{4})$/ =~ val
        yyyymmdd([yr1, 1, 1], [yr2, 12, 31])
      elsif /^(?<yr1>\d{4})-(?<mo1>\d\d?)-(?<yr2>\d{4})-(?<mo2>\d\d?)$/ =~ val
        yyyymmdd([yr1, mo1, 1], [yr2, mo2, 31])
      elsif /^(?<yr1>\d{4})-(?<mo1>\d\d?)-(?<dy1>\d\d?)-
             (?<yr2>\d{4})-(?<mo2>\d\d?)-(?<dy2>\d\d?)$/x =~ val
        yyyymmdd([yr1, mo1, dy1], [yr2, mo2, dy2])
      elsif /^(?<mo>\d\d?)$/ =~ val
        mmdd([mo, 1], [mo, 31])
      elsif /^(?<mo1>\d\d?)-(?<mo2>\d\d?)$/ =~ val
        mmdd([mo1, 1], [mo2, 31])
      elsif /^(?<mo1>\d\d?)-(?<dy1>\d\d?)-(?<mo2>\d\d?)-(?<dy2>\d\d?)$/ =~ val
        mmdd([mo1, dy1], [mo2, dy2])
      else
        raise BadDateRangeError.new(var: var, val: val)
      end

Each if or elsif has a named capture group that's used in the following conditionally executed code.
But if you re-write the above as a case statement, , e.g.:

      case val
      when /^(?<yr>\d{4})$/ 
        yyyymmdd([yr, 1, 1], [yr, 12, 31])
      ...
      end

then the named capture group (yr)is not available to the conditionally executed code.
This is due to the fact that when uses ===, which is not equivalent to the =~ used in if and elsif. Compare

$ /(?<yr>\d{4})/ === "20190815"; yr
true
nil

with

$ /(?<yr>\d{4})/ =~ "20190815"; yr
0
"2019"

Comment:
Some options:

1. Mark Style/CaseLikeIf as unsafe.
2. Make Style/CaseLikeIf safe by catching all the edge cases. Strikes me as very difficult or impossible to accomplish.
3. Make Style/CaseLikeIf safe by ignoring conditionals where an if or elsif includes =~ or .match. (I don't like this. Style/CaseLikeIf does work for the most common regex use case, and I wouldn't like to throw that away.)

RuboCop version

0.88.0 (using Parser 2.7.1.4, rubocop-ast 0.3.0, running on ruby 2.6.6 x86_64-linux)

[dmytro-savochkin]

Yeah, I already prepared a PR to fix this: #8504. Hopefully it will be merged soon. There I am going to prevent this cop from throwing an offense if there is a regexp with named captures inside and =~ or match is used. It isn't ideal though, it won't throw an offense if there is a regexp with named captures which are not used later. But probably this would be a very edgy case.