Proposed resolution:

Change [re.traits] p.12:

bool isctype(charT c, char_class_type f) const;

11 ...

12 Returns: Converts f into a value m of type std::ctype_base::mask in an unspecified manner, and returns true if use_facet<ctype<charT> >(getloc()).is(cm, mc) is true. Otherwise returns true if f bitwise or'ed with the result of calling lookup_classname with an iterator pair that designates the character sequence "w" is not equal to 0 and c == '_', or if f bitwise or'ed with the result of calling lookup_classname with an iterator pair that designates the character sequence "blank" is not equal to 0 and c is one of an implementation-defined subset of the characters for which isspace(c, getloc()) returns true, otherwise returns false.

[ Adopted at 2010-11 Batavia ]

[ Post-Rapperswil: ]

Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[re.traits]/12 describes regex_traits::isctype in terms of ctype::is(c, m), where c is a charT and m is a ctype_base::mask. Unfortunately [locale.ctype.members] specifies this function as ctype::is(m, c)