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PyUnicode_AsWideChar() doesn't merge surrogate pairs on a system with 32 bits wchar_t and Python compiled in narrow mode (sizeof(wchar_t) == 4 and sizeof(Py_UNICODE) == 2) => see issue #8670.
It is not easy to fix this problem because the callers of PyUnicode_AsWideChar() suppose that the output (wide character) string has the same length (in character) than the input (PyUnicode) string (suppose that sizeof(wchar_t) == sizeof(Py_UNICODE)). And PyUnicode_AsWideChar() doesn't write nul character at the end if the output string is truncated.
To prepare this change, a new PyUnicode_AsWideCharString() function would help because it does compute the size of the output buffer (whereas PyUnicode_AsWideChar() requires the output buffer in an argument).
Attached patch implements it:
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/* Convert the Unicode object to a wide character string. The output string
 always ends with a nul character. If size is not NULL, write the number of
 wide characters (including the final nul character) into *size.
 Returns a buffer allocated by PyMem_Alloc() (use PyMem_Free() to free it) on
 success. On error, returns NULL and *size is undefined. */
PyAPI_FUNC(wchar_t*) PyUnicode_AsWideCharString(
 PyUnicodeObject *unicode, /* Unicode object */
 Py_ssize_t *size /* number of characters of the result */
 );
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