apply simplecov fix in an overlay patch

spec/spec_helper.rb

@@ -15,6 +15,7 @@
 if (ENV['SKIP_SIMPLECOV'] != '1') && !RSpec.configuration.files_to_run.all? { |path| path.include?('/benchmark/') }
   # +SimpleCov.start+ must be invoked before any application code is loaded
   require 'simplecov'
+  require 'support/simplecov_fix'
   SimpleCov.start do
     formatter SimpleCov::Formatter::SimpleFormatter
   end

spec/support/simplecov_fix.rb

@@ -0,0 +1,30 @@
+# Overlay patch fix for https://github.com/simplecov-ruby/simplecov/pull/972.
+#
+# simplecov supports branch coverage only on some Ruby runtimes (for example,
+# not on JRuby). However, when merging coverage reports across runs,
+# simplecov assumes that branch coverage is always present.
+# This causes merging to fail when the runs are on different Ruby runtimes
+# as well as when merging on JRuby.
+#
+# Upstream has not fixed the issue in 3+ years (PR opened in 2021).
+#
+# dd-trace-rb has been using a fork of simplecov with this fix, however
+# this is awkward because bundler insists on constantly installing the fork
+# even though nothing in it or related to it changed.
+#
+# This file now brings the patch into our tree, permitting us to use any
+# released version of simplecov and removing the need to reference a fork.
+
+module SimpleCovCombineFix
+  def combine(coverage_a, coverage_b)
+    super.tap do |result|
+      if SimpleCov.branch_coverage?
+        result['branches'] ||= {}
+      end
+    end
+  end
+end
+
+class << SimpleCov::Combine::FilesCombiner
+  prepend(SimpleCovCombineFix)
+end