utf8.c: Make new static fcn more flexible

This commit allows this function to be called with NULL parameters when
the result of these is not needed.

embed.fnc

@@ -2500,8 +2500,8 @@ S	|void	|warn_on_first_deprecated_use				    \
 				|const unsigned line
 #endif
 S	|UV	|to_case_cp_list|const UV original				\
-				|NN const U32 ** const remaining_list		\
-				|NN Size_t * remaining_count			\
+				|NULLOK const U32 ** const remaining_list	\
+				|NULLOK Size_t * remaining_count		\
 				|NN SV *invlist					\
 				|NN const I32 * const invmap			\
 				|NULLOK const U32 * const * const aux_tables	\

proto.h

@@ -6670,7 +6670,7 @@ STATIC HV *	S_new_msg_hv(pTHX_ const char * const message, U32 categories, U32 f
 
 STATIC UV	S_to_case_cp_list(pTHX_ const UV original, const U32 ** const remaining_list, Size_t * remaining_count, SV *invlist, const I32 * const invmap, const U32 * const * const aux_tables, const U8 * const aux_table_lengths, const char * const normal);
 #define PERL_ARGS_ASSERT_TO_CASE_CP_LIST	\
-	assert(remaining_list); assert(remaining_count); assert(invlist); assert(invmap); assert(normal)
+	assert(invlist); assert(invmap); assert(normal)
 STATIC U8	S_to_lower_latin1(const U8 c, U8 *p, STRLEN *lenp, const char dummy)
 			__attribute__warn_unused_result__;
 #define PERL_ARGS_ASSERT_TO_LOWER_LATIN1

utf8.c

@@ -3178,10 +3178,16 @@ S_to_case_cp_list(pTHX_ const UV original,
     I32 base;
 
     /* Return the changed case of code point 'original'.  The first code point of
-     * the changed case is returned; *remaining_count will be set to how many
-     * other code points are in the changed case.  If it is non-zero,
-     * *remaining_list will point to a non-modifiable array containing them;
-     * if zero, *remaining_list is undefined.
+     * the changed case is returned.
+     *
+     * If 'remaining_count' is not NULL, *remaining_count will be set to how
+     * many other code points are in the changed case.  If non-zero and
+     * 'remaining_list' is also not NULL, *remaining_list will be set to point
+     * to a non-modifiable array containing the second and potentially third
+     * code points in the changed case.  (Unicode guarantees a maximum of 3.)
+     * Note that this means that *remaining_list is undefined unless there are
+     * multiple code points, and the caller has chosen to find out how many by
+     * making 'remaining_count' not NULL.
      *
      * 'normal' is a string to use to name the new case in any generated
      * messages, as a fallback if the operation being used is not available.
@@ -3197,6 +3203,11 @@ S_to_case_cp_list(pTHX_ const UV original,
     index = _invlist_search(invlist, original);
     base = invmap[index];
 
+    /* Most likely, the case change will contain just a single code point */
+    if (remaining_count) {
+        *remaining_count = 0;
+    }
+
     if (LIKELY(base == 0)) {    /* 0 => original was unchanged by casing */
 
         /* At this bottom level routine is where we warn about illegal code
@@ -3243,8 +3254,13 @@ S_to_case_cp_list(pTHX_ const UV original,
      * the first entry in the *remaining returns, as it is returned by the
      * function. */
     base = -base;
-    *remaining_list  = aux_tables[base] + 1;
-    *remaining_count = (Size_t) (aux_table_lengths[base] - 1);
+    if (remaining_count) {
+        *remaining_count = (Size_t) (aux_table_lengths[base] - 1);
+
+        if (remaining_list) {
+            *remaining_list  = aux_tables[base] + 1;
+        }
+    }
 
     return (UV) aux_tables[base][0];
 }