vim-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 vim/vim#9169, closes vim/vim#7486)
d85fccdfed

src/nvim/strings.c

@@ -1001,10 +1001,9 @@ int vim_vsnprintf_typval(char *str, size_t str_m, const char *fmt, va_list ap, t
                                  - str_arg);
           }
           if (fmt_spec == 'S') {
-            if (min_field_width != 0) {
+            size_t base_width = min_field_width;
-              min_field_width += (strlen(str_arg)
+            size_t pad_cell = 0;
-                                  - mb_string2cells((char_u *)str_arg));
+
-            }
             if (precision) {
               char_u *p1;
               size_t i = 0;
@@ -1016,7 +1015,11 @@ int vim_vsnprintf_typval(char *str, size_t str_m, const char *fmt, va_list ap, t
                   break;
                 }
               }
-              str_arg_l = (size_t)(p1 - (char_u *)str_arg);
+              pad_cell = min_field_width - precision;
+              base_width = str_arg_l = (size_t)(p1 - (char_u *)str_arg);
+            }
+            if (min_field_width != 0) {
+              min_field_width = base_width + pad_cell;
             }
           }
           break;

src/nvim/testdir/test_expr.vim

@@ -283,6 +283,11 @@ 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