Implementing p0515 - spaceship operator

[Jonathan Wakely]

t all cleaOn 11 January 2018 at 10:05, David Brown wrote:
> Maybe it is easier to say "gcc supports <=> in C++2a, and as an
> extension also supports it in C and C++ of any standard" ?  I don't
> believe there is any way for it to conflict with existing valid code, so
> it would do no harm as a gcc extension like that - and C users can then
> use it too.

It's not very useful in C because you need the comparison category
types, which are classes defined in <compare> (see
http://en.cppreference.com/w/cpp/header/compare)

C doesn't have those types, and can't define anything close.

And it's non-conforming to declare those types in pre-C++2a, because
the names could be used by user programs.

Potentially the types could be defined with reserved names like
__strong_ordering, and then make std::strong_ordering a typedef for
that, but there are also changes to the language spec that go with the
new operator, and enabling those pre-C++2a could change the meaning of
valid code.

So it's not ar it does no harm.