Limit format fields to more POSIX-like spec

* doc/lispref/strings.texi (Formatting Strings):
Don’t allow mixing numbered with unnumbered format specs.
* src/editfns.c (styled_format): Don’t bother checking for field 0,
since it doesn’t crash and the behavior is not specified.
* test/src/editfns-tests.el (format-with-field): Adjust tests to
match current doc.  Add more tests for out-of-range fields.

1 files changed, 4 insertions, 4 deletions

diff --git a/src/editfns.c b/src/editfns.c
index 1dbae8f5d4b..29af25aab4f 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -3900,8 +3900,10 @@ where field is [0-9]+ followed by a literal dollar "$", flags is
 [+ #-0]+, width is [0-9]+, and precision is a literal period "."
 followed by [0-9]+.
 
-If field is given, it must be a one-based argument number; the given
-argument is substituted instead of the next one.
+If a %-sequence is numbered with a field with positive value N, the
+Nth argument is substituted instead of the next one.  A format can
+contain either numbered or unnumbered %-sequences but not both, except
+that %% can be mixed with numbered %-sequences.
 
 The + flag character inserts a + before any positive number, while a
 space inserts a space before any positive number; these flags only
@@ -4081,8 +4083,6 @@ styled_format (ptrdiff_t nargs, Lisp_Object *args, bool message)
               num = str2num (format, &num_end);
               if (*num_end == '$')
                 {
-                  if (num == 0)
-                    error ("Invalid format field number 0");
                   n = num - 1;
                   format = num_end + 1;
                 }