Add optional disablement for pattern matching else branch

[bkuhlmann]

Overview

Hello. 👋 I'm not sure if this is a bug or a feature request but opting to go with the latter. I'm hitting a situation, with pattern matching, where I don't think I necessarily want code coverage for the else branch when I've configured SimpleCov as follows:

SimpleCov.start do
  enable_coverage :branch
  add_filter %r(^/spec/)
  minimum_coverage_by_file line: 95, branch: 95
end

Consider the following code:

module Demo
  include Dry::Monads[:result]

  def self.call result, logger: Logger.new(STDOUT)
    case result
      in Success(text) then logger.info text
      in Failure(error) then logger.error error
    end
  end
end

I'm using pattern matching and monads for a functional design. Situations in which I'd hit the else branch is low because a monad can only be a Success or Failure. It's not impossible, only very rare to hit the else branch. As an added benefit of pattern matching -- should the else branch be hit -- I'll get a NoMatchingPatternError exception so I'll know immediately if this situation occurs. That last statement makes pattern matching unique versus a traditional case...when statement and why I'm bringing this issue up.

One workaround -- and the most obvious -- is to always implement the else branch:

case result
  in Success(text) then logger.info text
  in Failure(error) then logger.error error
  else logger.fatal("Something unexpected happened.")
end

The other workaround is to ignore code coverage for the else branch entirely:

case result
  in Success(text) then logger.info text
  in Failure(error) then logger.error error
  # :nocov:
end

The only problem with either of these workarounds is that all of this gets tedious quickly for minimum benefit. 😢

Screenshots/Screencasts

Here's a screenshot of the report as generated by SimpleCov:

Steps to Recreate

To recreate, run the following:

1. Download this tiny demo.zip application.
2. Run unzip demo.zip.
3. Change directory to demo.
4. Run: bundle install
5. Run: bundle exec rspec
6. Run: open coverage/index.html

Environment

• OS: macOS 12.6.1 (21G217)
• Ruby: ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [arm64-darwin21.4.0]
• SimpleCov: 0.21.2

[Morozzzko]

Ran into the same issue and decided to see if there's a pull request or anything

Running macOS 14.2.1
ruby 3.2.3 (2024-01-18 revision 52bb2ac0a6) [x86_64-darwin23]
simpleov 0.22.0

My system shows ≈90%+ line coverage, but just ≈75% branch coverage. I was fairly surprised because I do test all significant branches, and those untested were mostly from generated boilerplate and weren't that important to test.
As it turns out, nearly all of those were from else branches in pattern matching

The thing is: we shouldn't care for an else branch for PM unless it's explicitly defined. A skilled developer would probably try and use PM with a comprehensive coverage – meaning there's no valid data that must be handled explicitly.
Writing tests for an absurd situation isn't particularly useful

So I'd like to have an option to handle that, too