Undefined behavior in locale_facets.tcc

[Doug Gregor]

Several routines in locale_facets.tcc use iterators passed to them in a 
way that invokes undefined behavior. The affected routines include:
	num_get::_M_extract_float
	num_get::_M_extract_int
	money_get::do_get
	time_get::_M_extract_num
	time_get::do_get_year

All of these have a loop that contains one or more occurrences of the 
expression *(++__beg), which invokes undefined behavior when ++__beg 
returns a past-the-end iterator (they can't be dereferenced).

However, we've never seen failures because:
	1) __beg is always an istreambuf_iterator (22.2.5/1, and others like 
it, say we're allowed to assume this).
	2) the libstdc++ istreambuf_iterator dereference operation returns 
traits_type::to_char_type(traits_type::eof()) when the iterator is 
past-the-end. This is an (undocumented) extension; 24.5.3/2 says the 
result is undefined.

The end result is that the code works for our istreambuf_iterators 
because of the extension, but won't necessary work for anything else. 
Is there a policy regarding reliance on libstdc++ extensions within 
libstdc++ code? This is especially important for me because under debug 
mode the standard-specified semantics are checked, so the use of the 
extension causes the debug mode to flag an error. How do we want to 
work around this problem? I/we could:

	1) Rewrite the offending code to not use the extension.
	2) Hide the past-the-end check for istreambuf_iterator behind 
_GLIBCPP_DEBUG_PEDANTIC (which flags errors at uses of extension 
semantics).
	3) Turn off checking around these blocks of code.

It's likely that this issue will come up again, so I would prefer to 
have a policy in place that says what we should do.

	Doug