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, 26 insertions, 0 deletions

diff --git a/src/character.c b/src/character.c
index fa817a50317..4a934c7801c 100644
--- a/src/character.c
+++ b/src/character.c
@@ -1050,6 +1050,32 @@ blankp (int c)
   return XINT (category) == UNICODE_CATEGORY_Zs; /* separator, space */
 }
 
+
+/* Return true for characters that would read as symbol characters,
+   but graphically may be confused with some kind of punctuation.  We
+   require an escaping backslash, when such characters begin a
+   symbol.  */
+bool
+confusable_symbol_character_p (int ch)
+{
+  switch (ch)
+    {
+    case 0x2018: /* LEFT SINGLE QUOTATION MARK */
+    case 0x2019: /* RIGHT SINGLE QUOTATION MARK */
+    case 0x201B: /* SINGLE HIGH-REVERSED-9 QUOTATION MARK */
+    case 0x201C: /* LEFT DOUBLE QUOTATION MARK */
+    case 0x201D: /* RIGHT DOUBLE QUOTATION MARK */
+    case 0x201F: /* DOUBLE HIGH-REVERSED-9 QUOTATION MARK */
+    case 0x301E: /* DOUBLE PRIME QUOTATION MARK */
+    case 0xFF02: /* FULLWIDTH QUOTATION MARK */
+    case 0xFF07: /* FULLWIDTH APOSTROPHE */
+      return true;
+
+    default:
+      return false;
+    }
+}
+
 signed char HEXDIGIT_CONST hexdigit[UCHAR_MAX + 1] =
   {
 #if HEXDIGIT_IS_CONST