gcc9 snapshot 20190414 is 30x slower than gcc 6.3

[Jason Mancini]

On gcc trunk, the performance culprit is at gcc/cp/call.c function build_over_call at:

if (undeduced_auto_decl (fn))
  mark_used (fn, complain); // <= this guy from gcc-7-branch r249333
else
  /* Otherwise set TREE_USED for the benefit of -Wunused-function. See PR80598.  */
  TREE_USED (fn) = 1;

I'm still working on a code sample.  The code sample has to be large to tickle the issue so far.