Fix reader char escape bugs (bug#55738)

Make the character literal ?\LF (linefeed) generate 10, not -1.

Ensure that Control escape sequences in character literals are
idempotent: ?\C-\C-a and ?\^\^a mean the same thing as ?\C-a and ?\^a,
generating the control character with value 1.  "\C-\C-a" no longer
signals an error.

* src/lread.c (read_escape): Make nonrecursive and only combine
the base char with modifiers at the end, creating control chars
if applicable.  Remove the `stringp` argument; assume character
literal syntax.  Never return -1.
(read_string_literal): Handle string-specific escape semantics here
and simplify.
* test/src/lread-tests.el (lread-misc-2): New test.

1 files changed, 10 insertions, 0 deletions

diff --git a/test/src/lread-tests.el b/test/src/lread-tests.el
index 47351c1d116..59d5ca076f1 100644
--- a/test/src/lread-tests.el
+++ b/test/src/lread-tests.el
@@ -317,4 +317,14 @@ literals (Bug#20852)."
   (should (equal (read-from-string "#_")
                  '(## . 2))))
 
+(ert-deftest lread-misc-2 ()
+  ;; ?\LF should produce LF (only inside string literals do we ignore \LF).
+  (should (equal (read-from-string "?\\\n") '(?\n . 3)))
+  (should (equal (read-from-string "\"a\\\nb\"") '("ab" . 6)))
+  ;; The Control modifier constructs should be idempotent.
+  (should (equal ?\C-\C-x ?\C-x))
+  (should (equal ?\^\^x ?\C-x))
+  (should (equal ?\C-\^x ?\C-x))
+  (should (equal ?\^\C-x ?\C-x)))
+
 ;;; lread-tests.el ends here