[gcc r13-4799] c++: ICE with concepts TS multiple auto deduction [PR101886]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:a7c8036b26082d8da001e05596777c5f911590e1

commit r13-4799-ga7c8036b26082d8da001e05596777c5f911590e1
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Dec 19 14:59:43 2022 -0500

    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.

Diff:
---
 gcc/cp/pt.cc                          | 12 +++++++++---
 gcc/testsuite/g++.dg/concepts/auto5.C |  9 +++++++++
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 2516cca590e..2b7b3756b68 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.  */
diff --git a/gcc/testsuite/g++.dg/concepts/auto5.C b/gcc/testsuite/g++.dg/concepts/auto5.C
new file mode 100644
index 00000000000..f1d653efd87
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/auto5.C
@@ -0,0 +1,9 @@
+// PR c++/101886
+// { dg-do compile { target c++17_only } }
+// { dg-options "-fconcepts-ts" }
+
+template<typename...> struct A { };
+
+A<int, int> a;
+A<auto, auto> b1 = a;
+A<auto, auto> b2 = a;