diff -r 25d97090c6ca -r 948fac24bc39 mercurial/interfaces/repository.py
--- a/mercurial/interfaces/repository.py	Sat Mar 07 00:30:33 2020 +0100
+++ b/mercurial/interfaces/repository.py	Thu Apr 02 13:04:41 2020 -0400
@@ -1052,6 +1052,9 @@
 
         Returns a 2-tuple containing ``bytearray(self.text())`` and the
         delta between ``base`` and this manifest.
+
+        If this manifest implementation can't support ``fastdelta()``,
+        raise ``mercurial.manifest.FastdeltaUnavailable``.
         """