Don't apply shorthands to punctuation-only symbols (bug#51089)

This includes symbols used for arithmetic functions such as -, /=,
etc.  Using "-" or "/=" is still possible but doing so won't shadow
those functions.

* doc/lispref/symbols.texi (Shorthand, Exceptions): New
subsubsection.

* src/lread.c (read1): Exempt punctionation-only symbols from
oblookup_considering_shorthand.

* test/lisp/progmodes/elisp-mode-tests.el
(elisp-dont-shadow-punctuation-only-symbols): Tweak test.

doc/lispref/symbols.texi

@@ -735,3 +735,20 @@ instead of @code{snu-}.
 ;;                          ("sns-" . "some-nice-string-utils-"))
 ;; End:
 @end example
+
+@subsection Exceptions
+
+There are two exceptions to rules governing Shorthand transformations:
+
+@itemize @bullet
+@item
+Symbol forms comprised entirely of symbol constituents (@pxref{Syntax
+Class Table}) are exempt not transform.  For example, it's possible to
+use @code{-} or @code{/=} as shorthand prefixes, but that won't shadow
+the arithmetic @emph{functions} that have exactly that prefix as their
+full name.;
+
+@item
+Symbol forms whose name starts with the the characters @code{#_} are
+also exempted.
+@end itemize

src/lread.c

@@ -3805,7 +3805,12 @@ read1 (Lisp_Object readcharfun, int *pch, bool first_in_list)
 	      ptrdiff_t longhand_bytes = 0;
 
 	      Lisp_Object tem;
-	      if (skip_shorthand)
+	      if (skip_shorthand ||
+		  /* The following ASCII characters are used in the
+		     only "core" Emacs Lisp symbols that are
+		     exclusively comprised of 'symbol constituent'
+		     syntax. */
+		  strspn(read_buffer, "^*+-/<=>_|") >= nbytes)
 		tem = oblookup (obarray, read_buffer, nchars, nbytes);
 	      else
 		tem = oblookup_considering_shorthand (obarray, read_buffer,

test/lisp/progmodes/elisp-mode-tests.el

@@ -1094,9 +1094,8 @@ evaluation of BODY."
     (should (unintern "f-test4---"))))
 
 (ert-deftest elisp-dont-shadow-punctuation-only-symbols ()
-  :expected-result :failed ;  bug#51089
-  (let* ((shorthanded-form '(- 42 (-foo 42)))
-         (expected-longhand-form '(- 42 (fooey-foo 42)))
+  (let* ((shorthanded-form '(/= 42 (-foo 42)))
+         (expected-longhand-form '(/= 42 (fooey-foo 42)))
          (observed (let ((read-symbol-shorthands
                           '(("-" . "fooey-"))))
                      (car (read-from-string