Undefined behavior in locale_facets.tcc

[Jerry Quinn <jlquinn at optonline dot net>]

Doug Gregor writes:

 > 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.

4) Change the standard :-)

The correct solution is probably to allow the extension for the
iterators we instantiate (char and wchar_t), and fix the code
otherwise.  Assuming the extension actually buys any speed.

How hard is it to build a testcase that will fail?

Jerry Quinn