Fix Fchar_syntax for non-ASCII in unibyte buffers

Fchar_syntax did not convert unibyte characters to multibyte when the
current buffer was unibyte, in contrast to `char-syntax` in
byte-compiled code (bug#53260).

* src/bytecode.c (exec_byte_code): Call out to Fchar_syntax;
the dynamic frequency is too low to justify inlining here, and it
did lead to implementations diverging.
* src/syntax.c (Fchar_syntax): Convert non-ASCII unibyte values to
multibyte.
* test/src/syntax-tests.el (syntax-char-syntax): New test.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/syntax.c b/src/syntax.c
index 9df878b8edf..13c36fdf3cd 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -1101,10 +1101,11 @@ this is probably the wrong function to use, because it can't take
 `syntax-after' instead.  */)
   (Lisp_Object character)
 {
-  int char_int;
   CHECK_CHARACTER (character);
-  char_int = XFIXNUM (character);
+  int char_int = XFIXNAT (character);
   SETUP_BUFFER_SYNTAX_TABLE ();
+  if (NILP (BVAR (current_buffer, enable_multibyte_characters)))
+    char_int = make_char_multibyte (char_int);
   return make_fixnum (syntax_code_spec[SYNTAX (char_int)]);
 }