Add a new function syntax-class-to-char

* doc/lispref/syntax.texi (Syntax Table Internals): Document it.
* src/syntax.c (Fsyntax_class_to_char): New function (bug#37452).

doc/lispref/syntax.texi

@@ -1047,6 +1047,11 @@ Given a syntax descriptor @var{desc} (a string), this function returns
 the corresponding raw syntax descriptor.
 @end defun
 
+@defun syntax-class-to-char syntax
+Given a raw syntax descriptor @var{syntax} (an integer), this function
+returns the corresponding syntax descriptor (a character).
+@end defun
+
 @defun syntax-after pos
 This function returns the raw syntax descriptor for the character in
 the buffer after position @var{pos}, taking account of syntax

etc/NEWS

@@ -2843,6 +2843,11 @@ customize them.
 
 * Lisp Changes in Emacs 28.1
 
++++
+** New function 'syntax-class-to-char'.
+This does the almost the opposite of 'string-to-syntax' -- it returns
+the syntax descriptor (a character) given a raw syntax descriptor.
+
 +++
 ** New function 'buffer-local-boundp'.
 This predicate says whether a symbol is bound in a specific buffer.

src/syntax.c

@@ -1109,6 +1109,23 @@ this is probably the wrong function to use, because it can't take
   return make_fixnum (syntax_code_spec[SYNTAX (char_int)]);
 }
 
+DEFUN ("syntax-class-to-char", Fsyntax_class_to_char,
+       Ssyntax_class_to_char, 1, 1, 0,
+       doc: /* Return the syntax char of CLASS, described by an integer.
+For example, if SYNTAX is word constituent (the integer 2), the
+character `w' (119) is returned.  */)
+  (Lisp_Object syntax)
+{
+  int syn;
+  CHECK_FIXNUM (syntax);
+  syn = XFIXNUM (syntax);
+
+  if (syn < 0 || syn >= sizeof syntax_code_spec)
+    args_out_of_range (make_fixnum (sizeof syntax_code_spec - 1),
+		       syntax);
+  return make_fixnum (syntax_code_spec[syn]);
+}
+
 DEFUN ("matching-paren", Fmatching_paren, Smatching_paren, 1, 1, 0,
        doc: /* Return the matching parenthesis of CHARACTER, or nil if none.  */)
   (Lisp_Object character)
@@ -3782,6 +3799,7 @@ In both cases, LIMIT bounds the search. */);
   defsubr (&Scopy_syntax_table);
   defsubr (&Sset_syntax_table);
   defsubr (&Schar_syntax);
+  defsubr (&Ssyntax_class_to_char);
   defsubr (&Smatching_paren);
   defsubr (&Sstring_to_syntax);
   defsubr (&Smodify_syntax_entry);

test/src/syntax-tests.el

@@ -21,6 +21,7 @@
 
 (require 'ert)
 (require 'ert-x)
+(require 'cl-lib)
 
 (ert-deftest parse-partial-sexp-continue-over-comment-marker ()
   "Continue a parse that stopped in the middle of a comment marker."
@@ -56,6 +57,16 @@
       (should (equal (parse-partial-sexp aftC pointX nil nil pps-aftC)
                      ppsX)))))
 
+(ert-deftest syntax-class-character-test ()
+  (cl-loop for char across " .w_()'\"$\\/<>@!|"
+           for i from 0
+           do (should (= char (syntax-class-to-char i)))
+           when (string-to-syntax (string char))
+           do (should (= char (syntax-class-to-char
+                               (car (string-to-syntax (string char)))))))
+  (should-error (syntax-class-to-char -1))
+  (should-error (syntax-class-to-char 200)))
+
 (ert-deftest parse-partial-sexp-paren-comments ()
   "Test syntax parsing with paren comment markers.
 Specifically, where the first character of the comment marker is