re PR c++/63924 (Constexpr constructible expression "is not constexpr" when used…

re PR c++/63924 (Constexpr constructible expression "is not constexpr" when used in a template non-type argument)

	PR c++/63924
	* constexpr.c (cxx_eval_constant_expression) [PARM_DECL]: A load
	from a variable of empty class type is constant.

From-SVN: r217749

gcc/cp/ChangeLog

 2014-11-18  Jason Merrill  <jason@redhat.com>
 
+	PR c++/63924
+	* constexpr.c (cxx_eval_constant_expression) [PARM_DECL]: A load
+	from a variable of empty class type is constant.
+
 	* constexpr.c (cxx_eval_statement_list): Handle statement-expressions.
 	(potential_constant_expression_1): Handle STMT_EXPR.
 

gcc/cp/constexpr.c

@@ -2845,6 +2845,12 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	r = *p;
       else if (addr)
 	/* Defer in case this is only used for its type.  */;
+      else if (is_empty_class (TREE_TYPE (t)))
+	{
+	  /* If the class is empty, we aren't actually loading anything.  */
+	  r = build_constructor (TREE_TYPE (t), NULL);
+	  TREE_CONSTANT (r) = true;
+	}
       else
 	{
 	  if (!ctx->quiet)

gcc/testsuite/g++.dg/cpp0x/constexpr-empty8.C

+// PR c++/63924
+// { dg-do compile { target c++11 } }
+
+struct A { };
+constexpr bool f(A a) { return true; }
+template <bool B> constexpr bool g() { return true; }
+constexpr bool g(A a) { return g<f(a)>(); }