DR 2137 - copy-constructor rank in list-initialization

* call.c (implicit_conversion): If we choose a copy constructor
	for list-initialization from the same type, the conversion is an
	exact match.

From-SVN: r235219

gcc/cp/ChangeLog

@@ -1,5 +1,10 @@
 2016-04-19  Jason Merrill  <jason@redhat.com>
 
+	DR 2137
+	* call.c (implicit_conversion): If we choose a copy constructor
+	for list-initialization from the same type, the conversion is an
+	exact match.
+
 	* constexpr.c (breaks): Handle EXIT_EXPR.
 	(cxx_eval_loop_expr): Handle COMPOUND_EXPR body.
 	(cxx_eval_constant_expression): Handle EXIT_EXPR, improve handling

gcc/cp/call.c

@@ -1862,7 +1862,24 @@ implicit_conversion (tree to, tree from, tree expr, bool c_cast_p,
 
       cand = build_user_type_conversion_1 (to, expr, flags, complain);
       if (cand)
-	conv = cand->second_conv;
+	{
+	  if (BRACE_ENCLOSED_INITIALIZER_P (expr)
+	      && CONSTRUCTOR_NELTS (expr) == 1
+	      && !is_list_ctor (cand->fn))
+	    {
+	      /* "If C is not an initializer-list constructor and the
+		 initializer list has a single element of type cv U, where U is
+		 X or a class derived from X, the implicit conversion sequence
+		 has Exact Match rank if U is X, or Conversion rank if U is
+		 derived from X."  */
+	      tree elt = CONSTRUCTOR_ELT (expr, 0)->value;
+	      tree elttype = TREE_TYPE (elt);
+	      if (reference_related_p (to, elttype))
+		return implicit_conversion (to, elttype, elt,
+					    c_cast_p, flags, complain);
+	    }
+	  conv = cand->second_conv;
+	}
 
       /* We used to try to bind a reference to a temporary here, but that
 	 is now handled after the recursive call to this function at the end

gcc/testsuite/g++.dg/DRs/dr2137-1.C

@@ -0,0 +1,20 @@
+// DR 2137
+// { dg-do run { target c++11 } }
+
+// Test that an initializer_list constructor beats the copy constructor.
+
+#include <initializer_list>
+
+bool ok = false;
+
+struct Q {
+  Q() = default;
+  Q(Q const&) = default;
+  Q(Q&&) = default;
+  Q(std::initializer_list<Q>) { ok = true; }
+};
+
+int main() {
+  Q x = Q { Q() };
+  if (!ok) __builtin_abort ();
+}

gcc/testsuite/g++.dg/DRs/dr2137-2.C

@@ -0,0 +1,21 @@
+// DR 2137
+// { dg-do link { target c++11 } }
+
+// Test that copying Q is better than converting to R.
+
+struct Q {
+  Q() { }
+  Q(const Q&) { }
+};
+
+struct R {
+  R(const Q&);
+};
+
+void f(Q) { }
+void f(R);
+
+int main()
+{
+  f({Q()});
+}