[3.12] gh-76511: Fix email.Message.as_string() for non-ASCII message with ASCII charset (GH-116125) (GH-116364)

(cherry picked from commit f97f25ef5dfcdfec0d9a359fd970abd139cf3428)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

3 files changed, 17 insertions, 2 deletions

diff --git a/Lib/email/generator.py b/Lib/email/generator.py
index 7ccbe10..c8056ad 100644
--- a/Lib/email/generator.py
+++ b/Lib/email/generator.py
@@ -243,7 +243,7 @@ class Generator:
                 # existing message.
                 msg = deepcopy(msg)
                 del msg['content-transfer-encoding']
-                msg.set_payload(payload, charset)
+                msg.set_payload(msg._payload, charset)
                 payload = msg.get_payload()
                 self._munge_cte = (msg['content-transfer-encoding'],
                                    msg['content-type'])
diff --git a/Lib/email/message.py b/Lib/email/message.py
index fe76958..a14cca5 100644
--- a/Lib/email/message.py
+++ b/Lib/email/message.py
@@ -340,7 +340,7 @@ class Message:
                 return
             if not isinstance(charset, Charset):
                 charset = Charset(charset)
-            payload = payload.encode(charset.output_charset)
+            payload = payload.encode(charset.output_charset, 'surrogateescape')
         if hasattr(payload, 'decode'):
             self._payload = payload.decode('ascii', 'surrogateescape')
         else:
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
index 2a23709..a373c53 100644
--- a/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@ -336,6 +336,21 @@ class TestMessageAPI(TestEmailBase):
         msg = email.message_from_bytes(source)
         self.assertEqual(msg.as_string(), expected)
 
+    def test_nonascii_as_string_with_ascii_charset(self):
+        m = textwrap.dedent("""\
+            MIME-Version: 1.0
+            Content-type: text/plain; charset="us-ascii"
+            Content-Transfer-Encoding: 8bit
+
+            Test if non-ascii messages with no Content-Transfer-Encoding set
+            can be as_string'd:
+            Föö bär
+            """)
+        source = m.encode('iso-8859-1')
+        expected = source.decode('ascii', 'replace')
+        msg = email.message_from_bytes(source)
+        self.assertEqual(msg.as_string(), expected)
+
     def test_nonascii_as_string_without_content_type_and_cte(self):
         m = textwrap.dedent("""\
             MIME-Version: 1.0