patch 8.2.3630: printf() with %S does not handle multi-byte correctly

Problem:    Printf() with %S does not handle multi-byte correctly.
Solution:   Count cells instead of bytes. (closes #9169, closes #7486)

src/strings.c

@@ -2137,14 +2137,15 @@ vim_vsnprintf_typval(
 			char *q = memchr(str_arg, '\0',
 				  precision <= (size_t)0x7fffffffL ? precision
 						       : (size_t)0x7fffffffL);
+
 			str_arg_l = (q == NULL) ? precision
 						      : (size_t)(q - str_arg);
 		    }
 		    if (fmt_spec == 'S')
 		    {
-			if (min_field_width != 0)
-			    min_field_width += STRLEN(str_arg)
-				     - mb_string2cells((char_u *)str_arg, -1);
+			size_t base_width = min_field_width;
+			size_t pad_cell = 0;
+
 			if (precision)
 			{
 			    char_u  *p1;
@@ -2157,8 +2158,12 @@ vim_vsnprintf_typval(
 				if (i > precision)
 				    break;
 			    }
-			    str_arg_l = precision = p1 - (char_u *)str_arg;
+			    pad_cell = min_field_width - precision;
+			    base_width = str_arg_l = precision =
+							p1 - (char_u *)str_arg;
 			}
+			if (min_field_width != 0)
+			    min_field_width = base_width + pad_cell;
 		    }
 		    break;
 

src/testdir/test_expr.vim

@@ -297,6 +297,11 @@ function Test_printf_misc()
   call assert_equal('🐍', printf('%.2S', '🐍🐍'))
   call assert_equal('', printf('%.1S', '🐍🐍'))
 
+  call assert_equal('[    あいう]', printf('[%10.6S]', 'あいうえお'))
+  call assert_equal('[  あいうえ]', printf('[%10.8S]', 'あいうえお'))
+  call assert_equal('[あいうえお]', printf('[%10.10S]', 'あいうえお'))
+  call assert_equal('[あいうえお]', printf('[%10.12S]', 'あいうえお'))
+
   call assert_equal('1%', printf('%d%%', 1))
 endfunc
 

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3630,
 /**/
     3629,
 /**/