Fix round tripping of read->print for symbols with strange quotes

Since 2017-07-22 "Signal error for symbol names with strange
quotes (Bug#2967)", symbol names beginning with certain quote
characters require an escaping backslash.  However, the corresponding
change for printing missed, so that (eq (read (prin1-to-string SYM))
SYM) does not give `t' for such symbols.
* src/character.c (confusable_symbol_character_p): New function,
extracted from test `read1'.
* src/lread.c (read1): Use it.
* src/print.c (print_object): Use it to print a backslash for symbols
starting with characters that `read1' requires to be escaped.
* test/src/print-tests.el (print-read-roundtrip): New test.
* etc/NEWS.26:
* etc/NEWS: Clarify the announcement for the earlier reader
change (Bug#30217).

1 files changed, 4 insertions, 0 deletions

diff --git a/test/src/print-tests.el b/test/src/print-tests.el
index 46368c69ada..01e65028bc7 100644
--- a/test/src/print-tests.el
+++ b/test/src/print-tests.el
@@ -58,5 +58,9 @@
                        (buffer-string))
                      "--------\n"))))
 
+(ert-deftest print-read-roundtrip ()
+  (let ((sym '\’bar))
+    (should (eq (read (prin1-to-string sym)) sym))))
+
 (provide 'print-tests)
 ;;; print-tests.el ends here