bug#16731: 24.3.50; Latin small letter sharp s is not considered lower-case

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Andreas Röhler <andreas.roehler@easy-emacs.de>,
>   16731@debbugs.gnu.org
> Date: Thu, 13 Feb 2014 08:37:45 -0500
> 
> > How will we then be able to distinguish between lower-case characters
> > that have no upcase variant and characters that are not lower-case
> > characters at all?
> 
> Right: to handle this, we need to distinguish characters that are
> lower-case without an uppercase variant from characters which are
> neither lowercase nor uppercase.
> 
> We could do that by saying that the upcase table should return nil or -1
> for ß, to indicate that the upcase version is "missing".  But such
> a change will probably require carefully revising "all" the code that
> uses those tables.

Right.  I can instead suggest a much less intrusive change below.  Its
only disadvantage is that if some user or Lisp program overrides the
standard case tables, and actually _wants_ some lower-case characters
behave as if they weren't, looking at the Unicode tables will undo
such customizations.  If this is a concern, perhaps we could compare
the case table with the standard value, and only use the Unicode
attributes when they are equal?

If the approach below is accepted, a related question is how to treat
letters whose category is Lt, i.e. "titlecase" -- do we consider such
letters upper case or don't we?

--- src/buffer.h~0	2014-01-01 09:46:07.000000000 +0200
+++ src/buffer.h	2014-02-13 18:27:32.225839000 +0200
@@ -1349,7 +1349,19 @@ downcase (int c)
 }
 
 /* True if C is upper case.  */
-INLINE bool uppercasep (int c) { return downcase (c) != c; }
+INLINE bool uppercasep (int c)
+{
+  Lisp_Object val;
+
+  if (downcase (c) != c)
+    return true;
+
+  if (NILP (Vunicode_category_table))
+    return false;
+
+  val = CHAR_TABLE_REF (Vunicode_category_table, c);
+  return INTEGERP (val) && XINT (val) == UNICODE_CATEGORY_Lu;
+}
 
 /* Upcase a character C known to be not upper case.  */
 INLINE int
@@ -1364,7 +1376,16 @@ upcase1 (int c)
 INLINE bool
 lowercasep (int c)
 {
-  return !uppercasep (c) && upcase1 (c) != c;
+  Lisp_Object val;
+
+  if (!uppercasep (c) && upcase1 (c) != c)
+    return true;
+
+  if (NILP (Vunicode_category_table))
+    return false;
+
+  val = CHAR_TABLE_REF (Vunicode_category_table, c);
+  return INTEGERP (val) && XINT (val) == UNICODE_CATEGORY_Ll;
 }
 
 /* Upcase a character C, or make no change if that cannot be done.  */