Implementing p0515 - spaceship operator

[Joseph Myers]

On Thu, 11 Jan 2018, 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.

As per previous IRC discussion, changing the lexing to support this 
pp-token can break valid code in previous standards, e.g. code 
concatenating <=> and >, then stringizing the result (the C++ proposal for 
adding this feature also notes some obscure cases where the character 
sequence <=> can actually occur as tokens, not just pp-tokens - 
"X<&Y::operator<=>" and "x+&operator<=>y" - so of course patches adding 
this feature should add testcases using such cases with older -std= 
options).

Changes to cpp_avoid_paste (so that preprocessed output does not put a 
pp-token starting with > immediately after <=) do not need to be 
conditional on the standard version, however.

-- 
Joseph S. Myers
joseph@codesourcery.com