Does :focus override :slow when both tags are present in RSpec 3.12?

Direct answer

No. In RSpec 3.12 (and all 3.x releases), an example tagged with both :focus and :slow will be excluded when filter_run_when_matching :focus and filter_run_excluding :slow are both active. Exclusion filters take precedence over inclusion filters regardless of configuration order or CLI flags.

Why this happens

RSpec merges all inclusion filters (from filter_run_when_matching, filter_run_including, and --tag focus) into a single inclusion set, and all exclusion filters (from filter_run_excluding and --tag ~slow) into a single exclusion set. The engine then applies the logic: run examples that match the inclusion set and do not match the exclusion set. Because :slow is in the exclusion set, any example carrying that tag is removed even if it also carries :focus.

Answers to your three sub-questions

1. Which filter wins? The exclusion filter (:slow) always wins.
2. Does configuration order matter? No. filter_run_when_matching :focus before or after filter_run_excluding :slow produces the same result.
3. How does rspec --tag ~slow combine? It adds another exclusion filter. The union of configured and CLI exclusions is applied, so --tag ~slow also excludes the :focus, :slow example.

Minimal verification steps

1. Create a spec file with two examples:

   # spec/filter_precedence_spec.rb
   RSpec.describe 'Filter precedence' do
     it 'runs when only focus', :focus do
       expect(true).to be true
     end
   
     it 'is excluded despite focus', :focus, :slow do
       expect(true).to be true
     end
   end
   

2. In spec_helper.rb (or rails_helper.rb):

   RSpec.configure do |config|
     config.filter_run_when_matching :focus
     config.filter_run_excluding :slow
   end
   

3. Run bundle exec rspec. Only the first example executes; the second is excluded.
4. Repeat with bundle exec rspec --tag ~slow (remove the configured exclusion). Same result — the :focus, :slow example is still excluded.

Workarounds if you need :focus to force-run a slow test

• Remove the :slow tag from that specific example.
• Temporarily disable the exclusion: bundle exec rspec --tag ~slow:false is not supported; instead, comment out filter_run_excluding :slow or run with a custom profile that omits it.
• Use a custom filter proc that checks for :focus first (advanced, rarely needed).

One diagnostic detail that could change the recommendation

Are you using the deprecated filter_run :focus (without _when_matching) anywhere in your configuration? That API behaves differently and can produce the inconsistent results mentioned in anecdotal reports.