gh-153862: Rebuild curses window.inch() from the wide cell

On a wide build inch() derived its result from winch(), whose character
is the whole code point rather than a locale byte, so a code point past
0xff overflowed into the color field and past 0xffff into the attribute
bits.  Read the cell with win_wch() and build the chtype from the locale
byte plus the attributes and color pair reported by getcchar().  The
character is 0 when it has no single-byte form; win_wch() and getcchar()
failures now raise.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Modules/_cursesmodule.c

@@ -776,30 +776,6 @@ curses_getcchar(const cchar_t *wcval, wchar_t *wstr, attr_t *attrs, int *pair)
     return rtn;
 }
 
-/* winch() does not convert the character to its locale byte, and its bits above
-   the character may be code-point bits rather than attributes, so rebuild the
-   value from the locale byte plus getcchar()'s attributes and color pair.  A
-   character with no single-byte form is left to winch(). */
-static chtype
-curses_cell_locale_byte(chtype rtn, const cchar_t *cell)
-{
-    wchar_t wstr[CCHARW_MAX + 1];
-    attr_t attrs;
-    int pair;
-    if (curses_getcchar(cell, wstr, &attrs, &pair) == ERR
-        || wstr[0] == L'\0' || wstr[1] != L'\0')
-    {
-        return rtn;
-    }
-    /* wctob() mirrors ncurses' own _nc_to_char(): the single-byte form, or EOF
-       when the character has none in this locale. */
-    int byte = wctob(wstr[0]);
-    if (byte != EOF) {
-        rtn = (unsigned char)byte | (attrs & ~(attr_t)A_COLOR) | COLOR_PAIR(pair);
-    }
-    return rtn;
-}
-
 /* Hash one cell by value (text, attributes, pair) -- consistent with the
    equality comparison, not the raw cchar_t whose padding and unused text tail
    it ignores.  Zero the key first so those bytes are deterministic, then
@@ -3650,7 +3626,40 @@ _curses_window_inch_impl(PyCursesWindowObject *self, int group_right_1,
 {
     chtype rtn;
     const char *funcname;
-
+#ifdef HAVE_NCURSESW
+    /* ncursesw's winch() returns the character's whole code point instead of
+       its locale byte, overflowing the chtype's 8-bit character field into the
+       color and attribute bits; read the wide cell and rebuild it instead. */
+    cchar_t cell = {0};
+    int rc;
+    if (!group_right_1) {
+        rc = win_wch(self->win, &cell);
+        funcname = "win_wch";
+    }
+    else {
+        rc = mvwin_wch(self->win, y, x, &cell);
+        funcname = "mvwin_wch";
+    }
+    if (rc == ERR) {
+        curses_window_set_error(self, funcname, "inch");
+        return NULL;
+    }
+    wchar_t wstr[CCHARW_MAX + 1];
+    attr_t attrs;
+    int pair;
+    if (curses_getcchar(&cell, wstr, &attrs, &pair) == ERR) {
+        curses_window_set_error(self, "getcchar", "inch");
+        return NULL;
+    }
+    int byte = 0;
+    if (wstr[0] != L'\0' && wstr[1] == L'\0') {
+        byte = wctob(wstr[0]);
+        if (byte == EOF) {
+            byte = 0;
+        }
+    }
+    rtn = (chtype)byte | (attrs & ~(attr_t)A_COLOR) | COLOR_PAIR(pair);
+#else
     if (!group_right_1) {
         rtn = winch(self->win);
         funcname = "winch";
@@ -3663,13 +3672,6 @@ _curses_window_inch_impl(PyCursesWindowObject *self, int group_right_1,
         curses_window_set_error(self, funcname, "inch");
         return NULL;
     }
-#ifdef HAVE_NCURSESW
-    curses_cell_t cell = {0};
-    if ((group_right_1 ? mvwin_wch(self->win, y, x, &cell)
-                       : win_wch(self->win, &cell)) != ERR)
-    {
-        rtn = curses_cell_locale_byte(rtn, &cell);
-    }
 #endif
     return PyLong_FromUnsignedLong(rtn);
 }