1 files changed, 27 insertions, 0 deletions
diff --git a/test/mkmf/test_config.rb b/test/mkmf/test_config.rb
index 7211c61d53..0d2cb3751c 100644
--- a/test/mkmf/test_config.rb
+++ b/test/mkmf/test_config.rb
@@ -1,5 +1,6 @@
 # frozen_string_literal: false
 require_relative 'base'
+require 'tempfile'
 
 class TestMkmfConfig < TestMkmf
   def test_dir_config
@@ -27,4 +28,30 @@ class TestMkmfConfig < TestMkmf
       assert_equal(false, with_config("foo"))
     end;
   end
+
+  def test_with_target_rbconfig
+    Tempfile.create(%w"rbconfig .rb", ".") do |tmp|
+      tmp.puts <<~'end;'
+      module RbConfig
+        CONFIG = {}
+        MAKEFILE_CONFIG = {}
+
+        def self.fire_update!(key, value); end
+        def self.expand(val, config = CONFIG); val; end
+      end;
+      ::RbConfig::CONFIG.each do |k, v|
+        tmp.puts "  CONFIG[#{k.dump}] = #{v.dump}"
+      end
+      ::RbConfig::MAKEFILE_CONFIG.each do |k, v|
+        tmp.puts "  MAKEFILE_CONFIG[#{k.dump}] = #{v.dump}"
+      end
+      tmp.puts "  CONFIG['testing-only'] = 'ok'"
+      tmp.puts "end"
+      tmp.close
+      assert_separately([], ["--target-rbconfig=#{tmp.path}"], <<-'end;')
+        assert_equal("ok", MakeMakefile::RbConfig::CONFIG["testing-only"])
+        assert_not_equal(::RbConfig, MakeMakefile::RbConfig)
+      end;
+    end
+  end
 end