#19063: fix set_payload handling of non-ASCII string input.

This version of the fix raises an error instead of accepting the invalid
input (ie: if a non-ASCII string is used but no charset is specified).

Lib/email/message.py

@@ -301,9 +301,23 @@ class Message:
         Optional charset sets the message's default character set.  See
         set_charset() for details.
         """
-        if isinstance(payload, bytes):
-            payload = payload.decode('ascii', 'surrogateescape')
-        self._payload = payload
+        if hasattr(payload, 'encode'):
+            if charset is None:
+                try:
+                    payload.encode('ascii', 'surrogateescape')
+                except UnicodeError:
+                    raise TypeError("charset argument must be specified"
+                        " when non-ASCII characters are used in the"
+                        " payload") from None
+                self._payload = payload
+                return
+            if not isinstance(charset, Charset):
+                charset = Charset(charset)
+            payload = payload.encode(charset.output_charset)
+        if hasattr(payload, 'decode'):
+            self._payload = payload.decode('ascii', 'surrogateescape')
+        else:
+            self._payload = payload
         if charset is not None:
             self.set_charset(charset)
 
@@ -342,7 +356,7 @@ class Message:
             try:
                 cte(self)
             except TypeError:
-                self._payload = charset.body_encode(self._payload)
+                self._payload = charset.body_encode(self.get_payload(decode=True))
                 self.add_header('Content-Transfer-Encoding', cte)
 
     def get_charset(self):