[PATCH] PR libstdc++/80624 satisfy invariant for char_traits<char16_t>::eof()

[Stephan Bergmann via libstdc++]

On 05/08/2017 12:19 PM, Jonathan Wakely via libstdc++ wrote:
> Changing the semantics of to_int_type for U+FFFF is far less likely to
> affect any ABIs (it's a constexpr function so it's possible somebody
> is using the value of to_int_type(char_type(-1)) as a template
> argument, but it seems unlikely. It's a much smaller change, "allowed"
> by http://www.unicode.org/faq/private_use.html#nonchar10 and it only
> affects a noncharacter that is not intended for interchange anyway.

I'm not convinced that a C++ implementation should be allowed to rewrite 
U+FFFF -> U+FFFD in this manner.  Noncharacters not being suitable for 
interchange IMO doesn't imply that an application shouldn't be able to 
write them out and read them back in.  (And 
<http://www.unicode.org/versions/corrigendum9.html> "Corrigendum #9: 
Clarification About Noncharacters" seems to support that view.)

To me, it feels like this change is not an improvement over keeping 
living with the current defect.