Make parse-partial-sexp signal an error if TO is smaller than FROM

* src/syntax.c (Fparse_partial_sexp): Signal an error if TO is
smaller than FROM (bug#49944).

1 files changed, 7 insertions, 2 deletions

diff --git a/src/syntax.c b/src/syntax.c
index 7bba336744a..6cbe0f6855f 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -3547,8 +3547,10 @@ DEFUN ("parse-partial-sexp", Fparse_partial_sexp, Sparse_partial_sexp, 2, 6, 0,
        doc: /* Parse Lisp syntax starting at FROM until TO; return status of parse at TO.
 Parsing stops at TO or when certain criteria are met;
  point is set to where parsing stops.
-If fifth arg OLDSTATE is omitted or nil,
- parsing assumes that FROM is the beginning of a function.
+
+If OLDSTATE is omitted or nil, parsing assumes that FROM is the
+ beginning of a function.  If not, OLDSTATE should be the state at
+ FROM.
 
 Value is a list of elements describing final state of parsing:
  0. depth in parens.
@@ -3594,6 +3596,9 @@ Sixth arg COMMENTSTOP non-nil means stop after the start of a comment.
   else
     target = TYPE_MINIMUM (EMACS_INT);  /* We won't reach this depth.  */
 
+  if (XFIXNUM (to) < XFIXNUM (from))
+    error ("End position should be larger than start position.");
+
   validate_region (&from, &to);
   internalize_parse_state (oldstate, &state);
   scan_sexps_forward (&state, XFIXNUM (from), CHAR_TO_BYTE (XFIXNUM (from)),