On IRC today the subject of why we don't have a strongly typed class for 
types rather than trees came up.

I did work on a prototype for this back in 2016/2017, but then got 
sidetracked by ranger.  It exists in an SVN branch and I briefly looked 
at resurrecting the patches a while ago and refitting them for trunk, 
but have not gotten back to it yet.  I have the patches on one of my 
machines too.

If there is interest, we could consider resurrecting it for GCC 15 stage 
1...   but regardless, I'll post the original PDF document here in case 
anyone is interested in how it worked.. back then anyway.

Andrew