[PATCH] c++: CTAD from initializer list [PR106366]

[Patrick Palka <ppalka@redhat.com>]

During CTAD, we currently perform the first phase of overload resolution
from [over.match.list] only if the class template has a list constructor.
But according to [over.match.class.deduct]/4 it should be enough to just
have a guide that looks like a list constructor (which is a more general
criterion in light of user-defined guides).

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

	PR c++/106366

gcc/cp/ChangeLog:

	* pt.cc (do_class_deduction): Don't consider TYPE_HAS_LIST_CTOR
	when setting try_list_ctor.  Reset args even when try_list_ctor
	is true and there are no list candidates.  Call resolve_args on
	the reset args.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction112.C: New test.
---
 gcc/cp/pt.cc                                  | 25 +++++++++----------
 .../g++.dg/cpp1z/class-deduction112.C         | 14 +++++++++++
 2 files changed, 26 insertions(+), 13 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction112.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 718dfa5bfa8..0f26d6f5bce 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -30250,8 +30250,8 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
   else if (BRACE_ENCLOSED_INITIALIZER_P (init))
     {
       list_init_p = true;
-      try_list_ctor = TYPE_HAS_LIST_CTOR (type);
-      if (try_list_ctor && CONSTRUCTOR_NELTS (init) == 1
+      try_list_ctor = true;
+      if (CONSTRUCTOR_NELTS (init) == 1
 	  && !CONSTRUCTOR_IS_DESIGNATED_INIT (init))
 	{
 	  /* As an exception, the first phase in 16.3.1.7 (considering the
@@ -30310,26 +30310,25 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
 
   tree fndecl = error_mark_node;
 
-  /* If this is list-initialization and the class has a list constructor, first
+  /* If this is list-initialization and the class has a list guide, first
      try deducing from the list as a single argument, as [over.match.list].  */
-  tree list_cands = NULL_TREE;
-  if (try_list_ctor && cands)
-    for (lkp_iterator iter (cands); iter; ++iter)
-      {
-	tree dg = *iter;
+  if (try_list_ctor)
+    {
+      tree list_cands = NULL_TREE;
+      for (tree dg : lkp_range (cands))
 	if (is_list_ctor (dg))
 	  list_cands = lookup_add (dg, list_cands);
-      }
-  if (list_cands)
-    {
-      fndecl = perform_dguide_overload_resolution (list_cands, args, tf_none);
-
+      if (list_cands)
+	fndecl = perform_dguide_overload_resolution (list_cands, args, tf_none);
       if (fndecl == error_mark_node)
 	{
 	  /* That didn't work, now try treating the list as a sequence of
 	     arguments.  */
 	  release_tree_vector (args);
 	  args = make_tree_vector_from_ctor (init);
+	  args = resolve_args (args, complain);
+	  if (args == NULL)
+	    return error_mark_node;
 	}
     }
 
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction112.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction112.C
new file mode 100644
index 00000000000..8da5868ff98
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction112.C
@@ -0,0 +1,14 @@
+// PR c++/106366
+// { dg-do compile { target c++17 } }
+
+#include <initializer_list>
+
+template<class T>
+struct A { A(...); };
+
+template<typename T>
+A(std::initializer_list<T>) -> A<T>;
+
+A a{1,2,3};
+using type = decltype(a);
+using type = A<int>;
-- 
2.37.1.208.ge72d93e88c