Improve documentation of 'copy-sequence'

* src/fns.c (Fcopy_sequence):
* doc/lispref/sequences.texi (Sequence Functions): Mention the
exception when copying an empty sequence.  (Bug#28627)

1 files changed, 4 insertions, 1 deletions

diff --git a/doc/lispref/sequences.texi b/doc/lispref/sequences.texi
index c7cf9f5e1af..5ae1567c128 100644
--- a/doc/lispref/sequences.texi
+++ b/doc/lispref/sequences.texi
@@ -155,7 +155,10 @@ This function generalizes @code{aref} (@pxref{Array Functions}) and
 @cindex copying sequences
 This function returns a copy of @var{seqr}, which should be either a
 sequence or a record.  The copy is the same type of object as the
-original, and it has the same elements in the same order.
+original, and it has the same elements in the same order.  However, if
+@var{seqr} is empty, like a string or a vector of zero length, the
+value returned by this function might not be a copy, but an empty
+object of the same type and identical to @var{seqr}.
 
 Storing a new element into the copy does not affect the original
 @var{seqr}, and vice versa.  However, the elements of the copy