vim-patch:9.1.0101: upper-case of German sharp s should be U+1E9E (#27449)

Problem:  upper-case of ß should be U+1E9E (CAPITAL LETTER SHARP S)
          (fenuks)
Solution: Make gU, ~ and g~ convert the U+00DF LATIN SMALL LETTER SHARP S (ß)
          to U+1E9E LATIN CAPITAL LETTER SHARP S (ẞ), update tests
          (glepnir)

This is part of Unicode 5.1.0 from April 2008, so should be fairly safe
to use now and since 2017 is part of the German standard orthography,
according to Wikipedia:
https://en.wikipedia.org/wiki/Capital_%E1%BA%9E#cite_note-auto-12

There is however one exception: UnicodeData.txt for U+00DF
LATIN SMALL LETTER SHARP S does NOT define U+1E9E LATIN CAPITAL LETTER
SHARP S as its upper case version. Therefore, toupper() won't be able
to convert from lower sharp s to upper case sharp s (the other way
around however works, since U+00DF is considered the lower case
character of U+1E9E and therefore tolower() works correctly for the
upper case version).

fixes: vim/vim#5573
closes: vim/vim#14018

https://github.com/vim/vim/commit/bd1232a1faf56b614a1e74c4ce51bc6e0650ae00

Co-authored-by: glepnir <glephunter@gmail.com>

2 files changed, 8 insertions, 5 deletions

diff --git a/src/nvim/mbyte.c b/src/nvim/mbyte.c
index fd353d8a67..f8451e62e2 100644
--- a/src/nvim/mbyte.c
+++ b/src/nvim/mbyte.c
@@ -1323,6 +1323,9 @@ int utf_fold(int a)
 // invalid values or can't handle latin1 when the locale is C.
 // Speed is most important here.
 
+// Note: UnicodeData.txt does not define U+1E9E as being the corresponding upper
+// case letter for U+00DF (ß), however it is part of the toLower table
+
 /// Return the upper-case equivalent of "a", which is a UCS-4 character.  Use
 /// simple case folding.
 int mb_toupper(int a)
diff --git a/src/nvim/ops.c b/src/nvim/ops.c
index d785be54e5..f91df49e4f 100644
--- a/src/nvim/ops.c
+++ b/src/nvim/ops.c
@@ -2165,16 +2165,16 @@ bool swapchar(int op_type, pos_T *pos)
     return false;
   }
 
-  if (op_type == OP_UPPER && c == 0xdf) {
+  // ~ is OP_NOP, g~ is OP_TILDE, gU is OP_UPPER
+  if ((op_type == OP_UPPER || op_type == OP_NOP || op_type == OP_TILDE) && c == 0xdf) {
     pos_T sp = curwin->w_cursor;
 
-    // Special handling of German sharp s: change to "SS".
+    // Special handling for lowercase German sharp s (ß): convert to uppercase (ẞ).
     curwin->w_cursor = *pos;
     del_char(false);
-    ins_char('S');
-    ins_char('S');
+    ins_char(0x1E9E);
     curwin->w_cursor = sp;
-    inc(pos);
+    return true;
   }
 
   int nc = c;