Proposed resolution:

In section 21.3.4, paragraph 1, change "data()[pos]" to "*(begin() + pos)".

Paraphrased from a message that Chris Newton posted to comp.std.c++:

The standard's description of basic_string<>::operator[] seems to violate const correctness.

The standard (21.3.4/1) says that "If pos < size(), returns data()[pos]." The types don't work. The return value of data() is const charT*, but operator[] has a non-const version whose return type is reference.