c++: NTTP constraint depending on outer parms [PR109160]

Here we're crashing during satisfaction for the NTTP 'C<B> auto V'
ultimately because convert_template_argument / unify don't pass all
outer template arguments to do_auto_deduction, and during satisfaction
we need to know all arguments.  While these callers do pass some outer
arguments, they are only sufficient to properly substitute the
(level-lowered) 'auto' and are not necessarily the entire set.

Fortunately it seems these callers have access to the full set of outer
arguments via convert_template_argument's 'in_decl' parameter and
unify's 'tparms' parameter.  So this patch adds a new parameter to
do_auto_deduction, used only during adc_unify deduction, through which
these callers can pass the enclosing (partially instantiated) template
and from which do_auto_deduction can obtain _all_ outer template
arguments for sake of satisfaction.

This patch also ensures that the 'in_decl' argument passed to
coerce_template_parms is always a TEMPLATE_DECL, which in turn allows us
to pass it as-is to do_auto_deduction; the only coerce_template_parms
caller that needed adjustment was tsubst_decl it seems.

	PR c++/109160

gcc/cp/ChangeLog:

	* cp-tree.h (do_auto_deduction): Add defaulted tmpl parameter.
	* pt.cc (convert_template_argument): Pass 'in_decl' as 'tmpl' to
	do_auto_deduction.
	(tsubst_decl) <case VAR_/TYPE_DECL>: Pass 'tmpl' instead of 't' as
	'in_decl' to coerce_template_parms.
	(unify) <case TEMPLATE_PARM_INDEX>: Pass TPARMS_PRIMARY_TEMPLATE
	as 'tmpl' to do_auto_deduction.
	(do_auto_deduction): Document default arguments.  Rename local
	variable 'tmpl' to 'ctmpl'.  Use 'tmpl' to obtain a full set of
	template arguments for satisfaction in the adc_unify case.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-placeholder12.C: New test.

2 files changed, 24 insertions, 11 deletions

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index d450b3d..622752a 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7326,7 +7326,8 @@ extern tree do_auto_deduction                   (tree, tree, tree,
                                                  auto_deduction_context
 						 = adc_unspecified,
 						 tree = NULL_TREE,
-						 int = LOOKUP_NORMAL);
+						 int = LOOKUP_NORMAL,
+						 tree = NULL_TREE);
 extern tree type_uses_auto			(tree);
 extern tree type_uses_auto_or_concept		(tree);
 extern void append_type_to_template_for_access_check (tree, tree, tree,
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 022f295..4429ae6 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -8644,7 +8644,7 @@ convert_template_argument (tree parm,
       else if (tree a = type_uses_auto (t))
 	{
 	  t = do_auto_deduction (t, arg, a, complain, adc_unify, args,
-				 LOOKUP_IMPLICIT);
+				 LOOKUP_IMPLICIT, /*tmpl=*/in_decl);
 	  if (t == error_mark_node)
 	    return error_mark_node;
 	}
@@ -15252,7 +15252,7 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
 		     the template.  */
 		  argvec = (coerce_template_parms
 			    (DECL_TEMPLATE_PARMS (gen_tmpl),
-			     argvec, t, complain));
+			     argvec, tmpl, complain));
 		if (argvec == error_mark_node)
 		  RETURN (error_mark_node);
 		hash = spec_hasher::hash (gen_tmpl, argvec);
@@ -24730,7 +24730,9 @@ unify (tree tparms, tree targs, tree parm, tree arg, int strict,
 	  if (tree a = type_uses_auto (tparm))
 	    {
 	      tparm = do_auto_deduction (tparm, arg, a,
-					 complain, adc_unify, targs);
+					 complain, adc_unify, targs,
+					 LOOKUP_NORMAL,
+					 TPARMS_PRIMARY_TEMPLATE (tparms));
 	      if (tparm == error_mark_node)
 		return 1;
 	    }
@@ -30779,13 +30781,20 @@ unparenthesized_id_or_class_member_access_p (tree init)
    adc_requirement contexts to communicate the necessary template arguments
    to satisfaction.  OUTER_TARGS is ignored in other contexts.
 
-   For partial-concept-ids, extra args may be appended to the list of deduced
-   template arguments prior to determining constraint satisfaction.  */
+   Additionally for adc_unify contexts TMPL is the template for which TYPE
+   is a template parameter type.
+
+   For partial-concept-ids, extra args from OUTER_TARGS, TMPL and the current
+   scope may be appended to the list of deduced template arguments prior to
+   determining constraint satisfaction as appropriate.  */
 
 tree
 do_auto_deduction (tree type, tree init, tree auto_node,
-                   tsubst_flags_t complain, auto_deduction_context context,
-		   tree outer_targs, int flags)
+		   tsubst_flags_t complain /* = tf_warning_or_error */,
+		   auto_deduction_context context /* = adc_unspecified */,
+		   tree outer_targs /* = NULL_TREE */,
+		   int flags /* = LOOKUP_NORMAL */,
+		   tree tmpl /* = NULL_TREE */)
 {
   if (init == error_mark_node)
     return error_mark_node;
@@ -30821,9 +30830,9 @@ do_auto_deduction (tree type, tree init, tree auto_node,
 					/*return*/true)))
     init = r;
 
-  if (tree tmpl = CLASS_PLACEHOLDER_TEMPLATE (auto_node))
+  if (tree ctmpl = CLASS_PLACEHOLDER_TEMPLATE (auto_node))
     /* C++17 class template argument deduction.  */
-    return do_class_deduction (type, tmpl, init, flags, complain);
+    return do_class_deduction (type, ctmpl, init, flags, complain);
 
   if (init == NULL_TREE || TREE_TYPE (init) == NULL_TREE)
     /* Nothing we can do with this, even in deduction context.  */
@@ -30975,7 +30984,10 @@ do_auto_deduction (tree type, tree init, tree auto_node,
 		}
 	    }
 
-      tree full_targs = add_to_template_args (outer_targs, targs);
+      tree full_targs = outer_targs;
+      if (context == adc_unify && tmpl)
+	full_targs = add_outermost_template_args (tmpl, full_targs);
+      full_targs = add_to_template_args (full_targs, targs);
 
       /* HACK: Compensate for callers not always communicating all levels of
 	 outer template arguments by filling in the outermost missing levels