From f104525c71fbd6338e98050201253f2fe464ec9b Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@ruby-lang.org>
Date: Thu, 28 Jan 2021 23:14:30 +0900
Subject: mkmf: cpp_command in C++ mode

---
 lib/mkmf.rb                 | 16 +++++++++++++++-
 test/mkmf/test_egrep_cpp.rb | 12 ++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/lib/mkmf.rb b/lib/mkmf.rb
index 38a5a15fb5..8db4647a40 100644
--- a/lib/mkmf.rb
+++ b/lib/mkmf.rb
@@ -573,11 +573,16 @@ MSG
                      conf)
   end
 
-  def cpp_command(outfile, opt="")
+  def cpp_config(opt)
     conf = cc_config(opt)
     if $universal and (arch_flag = conf['ARCH_FLAG']) and !arch_flag.empty?
       conf['ARCH_FLAG'] = arch_flag.gsub(/(?:\G|\s)-arch\s+\S+/, '')
     end
+    conf
+  end
+
+  def cpp_command(outfile, opt="")
+    conf = cpp_config(opt)
     RbConfig::expand("$(CPP) #$INCFLAGS #$CPPFLAGS #$CFLAGS #{opt} #{CONFTEST_C} #{outfile}",
                      conf)
   end
@@ -3037,6 +3042,15 @@ realclean: distclean
                        conf)
     end
 
+    def cpp_command(outfile, opt="")
+      conf = cpp_config(opt)
+      cpp = conf['CPP'].sub(/(\A|\s)#{Regexp.quote(conf['CC'])}(?=\z|\s)/) {
+        "#$1#{conf['CXX']}"
+      }
+      RbConfig::expand("#{cpp} #$INCFLAGS #$CPPFLAGS #$CXXFLAGS #{opt} #{CONFTEST_CXX} #{outfile}",
+                       conf)
+    end
+
     def link_command(ldflags, *opts)
       conf = link_config(ldflags, *opts)
       RbConfig::expand(TRY_LINK_CXX.dup, conf)
diff --git a/test/mkmf/test_egrep_cpp.rb b/test/mkmf/test_egrep_cpp.rb
index 7ac0e60010..2d10140368 100644
--- a/test/mkmf/test_egrep_cpp.rb
+++ b/test/mkmf/test_egrep_cpp.rb
@@ -10,4 +10,16 @@ class TestMkmfEgrepCpp < TestMkmf
   def test_not_have_func
     assert_equal(false, egrep_cpp(/never match/, ""), MKMFLOG)
   end
+
+  class TestMkmfEgrepCxx < self
+    def test_cxx_egrep_cpp
+      assert_equal(true, MakeMakefile["C++"].egrep_cpp(/^ok/, <<~SRC), MKMFLOG)
+      #ifdef __cplusplus
+      ok
+      #else
+      #error not C++
+      #endif
+      SRC
+    end
+  end
 end
-- 
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