From 6cad8c86b86637e9987a02cf84bddb1e8e397319 Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Wed, 26 Jul 2017 13:39:31 -0400
Subject: [PATCH] P0702R1 - List deduction of vector.

	* pt.c (do_class_deduction): Special-case deduction from a single
	element of related type.

From-SVN: r250584
---
 gcc/cp/ChangeLog                              |  6 ++++++
 gcc/cp/pt.c                                   | 18 ++++++++++++++++++
 .../g++.dg/cpp1z/class-deduction42.C          | 19 +++++++++++++++++++
 3 files changed, 43 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction42.C

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index b2df1eff896..9e763ae6d62 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,9 @@
+2017-07-26  Jason Merrill  <jason@redhat.com>
+
+	P0702R1 - List deduction of vector.
+	* pt.c (do_class_deduction): Special-case deduction from a single
+	element of related type.
+
 2017-07-26  Leonid Koppel  <lkoppel@uwaterloo.ca>
 
         PR c++/67054 - Inherited ctor with non-default-constructible members
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index bb323534ec0..173ec3f303b 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -25331,6 +25331,24 @@ do_class_deduction (tree ptype, tree tmpl, tree init, int flags,
   else if (BRACE_ENCLOSED_INITIALIZER_P (init))
     {
       try_list_ctor = TYPE_HAS_LIST_CTOR (type);
+      if (try_list_ctor && CONSTRUCTOR_NELTS (init) == 1)
+	{
+	  /* As an exception, the first phase in 16.3.1.7 (considering the
+	     initializer list as a single argument) is omitted if the
+	     initializer list consists of a single expression of type cv U,
+	     where U is a specialization of C or a class derived from a
+	     specialization of C.  */
+	  tree elt = CONSTRUCTOR_ELT (init, 0)->value;
+	  tree etype = TREE_TYPE (elt);
+
+	  tree tparms = INNERMOST_TEMPLATE_PARMS (DECL_TEMPLATE_PARMS (tmpl));
+	  tree targs = make_tree_vec (TREE_VEC_LENGTH (tparms));
+	  int err = unify (tparms, targs, type, etype,
+			   UNIFY_ALLOW_DERIVED, /*explain*/false);
+	  if (err == 0)
+	    try_list_ctor = false;
+	  ggc_free (targs);
+	}
       if (try_list_ctor || is_std_init_list (type))
 	args = make_tree_vector_single (init);
       else
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction42.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction42.C
new file mode 100644
index 00000000000..8217fd4d79a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction42.C
@@ -0,0 +1,19 @@
+// { dg-options -std=c++1z }
+
+#include <initializer_list>
+
+template <class,class> struct same;
+template <class T> struct same<T,T> { };
+
+template <class T>
+struct A
+{
+  A(const A&);
+  A(std::initializer_list<T>);
+};
+
+A a { 1 };
+A b { a };
+
+same<decltype (a), decltype (b)> s;
+
-- 
2.30.2