PR c++/72849 - ICE with incomplete class.

	* constexpr.c (cxx_eval_constant_expression): Check
	COMPLETE_TYPE_P before calling is_really_empty_class.
	* class.c (is_really_empty_class): Don't call complete_type.

From-SVN: r239289

gcc/cp/ChangeLog

 2016-08-09  Jason Merrill  <jason@redhat.com>
 
+	PR c++/72849
+	* constexpr.c (cxx_eval_constant_expression): Check
+	COMPLETE_TYPE_P before calling is_really_empty_class.
+	* class.c (is_really_empty_class): Don't call complete_type.
+
 	PR c++/56701
 	* typeck.c (cp_build_addr_expr_1): Remove special *this handling.
 

gcc/cp/class.c

@@ -8419,7 +8419,7 @@ is_really_empty_class (tree type)
 
       /* CLASSTYPE_EMPTY_P isn't set properly until the class is actually laid
 	 out, but we'd like to be able to check this before then.  */
-      if (COMPLETE_TYPE_P (complete_type (type)) && is_empty_class (type))
+      if (COMPLETE_TYPE_P (type) && is_empty_class (type))
 	return true;
 
       for (binfo = TYPE_BINFO (type), i = 0;

gcc/cp/constexpr.c

@@ -3699,7 +3699,8 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	 CONST_DECL for aggregate constants.  */
       if (lval)
 	return t;
-      if (is_really_empty_class (TREE_TYPE (t)))
+      if (COMPLETE_TYPE_P (TREE_TYPE (t))
+	  && is_really_empty_class (TREE_TYPE (t)))
 	{
 	  /* If the class is empty, we aren't actually loading anything.  */
 	  r = build_constructor (TREE_TYPE (t), NULL);

gcc/testsuite/g++.dg/template/incomplete7.C

+// PR c++/72849
+
+extern struct Foo a;
+template <typename> void fn1() { a; }