In my interpretation of the C standard (which I am reading from http://std.dkuug.dk/JTC1/SC22/WG14/www/docs/n843.pdf) UTF-8 is not a valid wchar_t encoding if your execution character set contains characters outside the C0 controls and Basic Latin range, and UTF-16 is not a valid wchar_t encoding if your execution character set has characters outside the BMP. In other words whatever you consider to be a character (which may be a combining character) must be encoded in one wchar_t code unit.
The relevant passage is 11 A wide character constant has type wchar_t, an integer type defined in the <stddef.h> header. The value of a wide character constant containing a single multibyte character that maps to a member of the extended execution character set is the wide character (code) corresponding to that multibyte character, as defined by the mbtowc function, with an implementation-defined current locale. The value of a wide character constant containing more than one multibyte character, or containing a multibyte character or escape sequence not represented in the extended execution character set, is implementation-defined. Tim
