c++: ICE with concepts TS multiple auto deduction [PR101886]

In extract_autos_r, we need to recompute TYPE_CANONICAL for the template
type parameter after adjusting its index, otherwise we end up with a
comptypes ICE for the below testcase.  Note that such in-place type
adjustment isn't generally safe to do since the type could be the
TYPE_CANONICAL of another (unadjusted) type, but in this case the
canonical auto (of some level and 0 index) is the first auto (of that
level) that's created, and so any auto that we do end up adjusting can't
be the canonical one.

	PR c++/101886

gcc/cp/ChangeLog:

	* pt.cc (extract_autos_r): Recompute TYPE_CANONICAL after
	adjusting the template type parameter's index.  Simplify
	by using TEMPLATE_TYPE_IDX.  Add some sanity checks.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/auto5.C: New test.

1 files changed, 9 insertions, 3 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 2516cca..2b7b375 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -29243,18 +29243,24 @@ extract_autos_r (tree t, void *data)
     {
       /* All the autos were built with index 0; fix that up now.  */
       tree *p = hash.find_slot (t, INSERT);
-      unsigned idx;
+      int idx;
       if (*p)
 	/* If this is a repeated constrained-type-specifier, use the index we
 	   chose before.  */
-	idx = TEMPLATE_PARM_IDX (TEMPLATE_TYPE_PARM_INDEX (*p));
+	idx = TEMPLATE_TYPE_IDX (*p);
       else
 	{
 	  /* Otherwise this is new, so use the current count.  */
 	  *p = t;
 	  idx = hash.elements () - 1;
 	}
-      TEMPLATE_PARM_IDX (TEMPLATE_TYPE_PARM_INDEX (t)) = idx;
+      if (idx != TEMPLATE_TYPE_IDX (t))
+	{
+	  gcc_checking_assert (TEMPLATE_TYPE_IDX (t) == 0);
+	  gcc_checking_assert (TYPE_CANONICAL (t) != t);
+	  TEMPLATE_TYPE_IDX (t) = idx;
+	  TYPE_CANONICAL (t) = canonical_type_parameter (t);
+	}
     }
 
   /* Always keep walking.  */