[3.15] gh-153862: Fix spurious color pair in curses window.inch() on a wide build (GH-154703) (GH-154720)

winch() returns the whole code point, so inch() replacing only its low
8 bits left the high bits in the color field.  Rebuild from
getcchar()'s attributes and color pair.

(cherry picked from commit b618874)

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

* gh-153862: Suggest instr() for non-encodable characters read by inch()

A character that does not fit in a single byte has a character byte of 0
in the value returned by inch(); document reading such characters with
instr().

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

---------

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

Author: serhiy-storchaka

Doc/library/curses.rst

@@ -1081,6 +1081,9 @@ Window objects
    and the color pair with :func:`pair_number`.
    The character byte is the locale-encoded byte of the cell's character,
    consistent with :meth:`instr`.
+   On a wide-character build, a character that does not fit in a single byte
+   in the current locale has a character byte of ``0``;
+   use :meth:`instr` to read such characters.
 
 
 .. method:: window.insch(ch[, attr])

Lib/test/test_curses.py

@@ -520,7 +520,7 @@ def test_read_from_window(self):
             with self.subTest(ch=ch):
                 stdscr.addstr(2, 0, ch)
                 self.assertEqual(stdscr.instr(2, 0, 1), b)
-                self.assertEqual(stdscr.inch(2, 0) & curses.A_CHARTEXT, b[0])
+                self.assertEqual(stdscr.inch(2, 0), b[0])
 
     def test_coordinate_errors(self):
         # Addressing a cell outside the window raises curses.error.

Modules/_cursesmodule.c

@@ -2102,34 +2102,6 @@ _curses_window_insch_impl(PyCursesWindowObject *self, int group_left_1,
     return curses_window_check_err(self, rtn, funcname, "insch");
 }
 
-#ifdef HAVE_NCURSESW
-/* winch() returns the low 8 bits of the character's code point with no locale
-   conversion, unlike instr(), so recover the locale byte from the wide cell
-   when the character maps to exactly one byte, keeping the attribute and color
-   bits in RTN.  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;
-    short pair;
-    /* getcchar() is not guaranteed to write the text of an empty cell. */
-    wstr[0] = L'\0';
-    if (getcchar(cell, wstr, &attrs, &pair, NULL) == 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 = (rtn & ~(chtype)A_CHARTEXT) | (unsigned char)byte;
-    }
-    return rtn;
-}
-#endif
-
 /*[clinic input]
 _curses.window.inch
 
@@ -2154,7 +2126,42 @@ _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;
+    short pair;
+    /* getcchar() is not guaranteed to write the text of an empty cell. */
+    wstr[0] = L'\0';
+    if (getcchar(&cell, wstr, &attrs, &pair, NULL) == 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";
@@ -2167,13 +2174,6 @@ _curses_window_inch_impl(PyCursesWindowObject *self, int group_right_1,
         curses_window_set_error(self, funcname, "inch");
         return NULL;
     }
-#ifdef HAVE_NCURSESW
-    cchar_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);
 }