Phobos and older DMD versions

[qznc]

My recent pull request [0] triggered some issue, which should get 
a bigger discussion:

Should Phobos version N also compile with DMD N-1?

In my opinion it should, because it makes Phobos development 
simpler. You don't need a self compiled dmd to fix bugs.

However, reality sometimes requires exceptions. I understand that 
sometimes Phobos uses a feature of dmd HEAD. That is why I said 
"should", but not "must". ;)


[0] 
https://github.com/D-Programming-Language/phobos/pull/1454/files#r5601514