Account for Windows console stdin encoding in test_stdinHEADmaster

On Windows default_external is UTF-8, but an interactive console STDIN
is read in the locale (console code page) encoding and transcoded to
the default external encoding. The previous assertion held only when
stdin was redirected, so it failed on a console with a non-UTF-8 code
page. Expect the locale encoding when stdin is a tty.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

1 files changed, 11 insertions, 2 deletions

diff --git a/test/ruby/test_io_m17n.rb b/test/ruby/test_io_m17n.rb
index 83d4fb0c7b..1736d01f78 100644
--- a/test/ruby/test_io_m17n.rb
+++ b/test/ruby/test_io_m17n.rb
@@ -404,8 +404,17 @@ EOT
   end
 
   def test_stdin
-    assert_equal(Encoding.default_external, STDIN.external_encoding)
-    assert_equal(nil, STDIN.internal_encoding)
+    encoding = Encoding.default_external
+    internal = nil
+    if /mswin|mingw/ =~ RUBY_PLATFORM and STDIN.tty?
+      # Interactive console input on Windows is read in the locale (console
+      # code page) encoding and transcoded to the default external encoding.
+      encoding = Encoding.find("locale")
+      internal = Encoding.default_internal || Encoding.default_external
+      internal = nil if internal == encoding
+    end
+    assert_equal(encoding, STDIN.external_encoding)
+    assert_equal(internal, STDIN.internal_encoding)
   end
 
   def test_stdout