Core issue 2310 - conversion to base of incomplete type.

	* class.c (build_base_path): Check COMPLETE_TYPE_P for source type.

From-SVN: r260127

gcc/cp/ChangeLog

 2018-05-09  Jason Merrill  <jason@redhat.com>
 
+	Core issue 2310 - conversion to base of incomplete type.
+	* class.c (build_base_path): Check COMPLETE_TYPE_P for source type.
+
 	CWG 2267 - list-initialization of reference temporary
 	* call.c (reference_binding): List-initializing a reference
 	temporary is copy-list-initialization.

gcc/cp/class.c

@@ -370,6 +370,15 @@ build_base_path (enum tree_code code,
       goto indout;
     }
 
+  if (!COMPLETE_TYPE_P (probe))
+    {
+      if (complain & tf_error)
+	error ("cannot convert from %qT to base class %qT because %qT is "
+	       "incomplete", BINFO_TYPE (d_binfo), BINFO_TYPE (binfo),
+	       BINFO_TYPE (d_binfo));
+      return error_mark_node;
+    }
+
   /* If we're in an NSDMI, we don't have the full constructor context yet
      that we need for converting to a virtual base, so just build a stub
      CONVERT_EXPR and expand it later in bot_replace.  */

gcc/testsuite/g++.dg/cpp0x/constexpr-base6.C

+// CWG issue 2310
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+template<typename A, typename B> struct check_derived_from { 
+  static A a; 
+  static constexpr B *p = &a;	// { dg-error "" }
+  int ar[p-p+1];
+}; 
+struct W { int i; }; 
+struct Z : W
+{
+  check_derived_from<Z, W> cdf;
+};