[newlib-cygwin/main] Cygwin: _sys_wcstombs: add FIXME comment

[Corinna Vinschen]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=ada3b8f7e5a71db4881135380c407a86a08f6663

commit ada3b8f7e5a71db4881135380c407a86a08f6663
Author:     Corinna Vinschen <corinna@vinschen.de>
AuthorDate: Wed Jul 23 22:42:52 2025 +0200
Commit:     Corinna Vinschen <corinna@vinschen.de>
CommitDate: Thu Jul 24 11:23:27 2025 +0200

    Cygwin: _sys_wcstombs: add FIXME comment
    
    Add a FIXME comment to the conversion of private use area bytes to
    "normal" bytes in _sys_wcstombs detailing the conversion difference
    between _sys_wcstombs and standard wcstombs.  It might be a good idea
    to drop the conversion to gather compatibility with wcstombs.
    
    Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

Diff:
---
 winsup/cygwin/strfuncs.cc | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/strfuncs.cc b/winsup/cygwin/strfuncs.cc
index caaf6b786295..eb6576051d90 100644
--- a/winsup/cygwin/strfuncs.cc
+++ b/winsup/cygwin/strfuncs.cc
@@ -965,8 +965,17 @@ _sys_wcstombs (char *dst, size_t len, const wchar_t *src, size_t nwc,
 
       /* Convert UNICODE private use area.  Reverse functionality for the
 	 ASCII area <= 0x7f (only for path names) is transform_chars above.
+
 	 Reverse functionality for invalid bytes in a multibyte sequence is
-	 in _sys_mbstowcs below. */
+	 in _sys_mbstowcs below.
+
+	 FIXME? The conversion of invalid bytes from the private use area
+	 like we do here is not actually necessary.  If we skip it, the
+	 generated multibyte string is not identical to the original multibyte
+	 string, but it's equivalent in the sense, that another mbstowcs will
+	 generate the same wide-char string.  It would also be identical to
+	 the same string converted by wcstombs.  And while the original
+	 multibyte string can't be converted by mbstowcs, this string can. */
       if (is_path && (pw & 0xff00) == 0xf000
 	  && (((cwc = (pw & 0xff)) <= 0x7f && tfx_rev_chars[cwc] >= 0xf000)
 	      || (cwc >= 0x80 && MB_CUR_MAX > 1)))