[backport gcc-4.8/trunk r194316, there was no ChangeLog entry
 for the new test case <mikpe@it.uu.se>]

gcc/cp/

2012-12-07  Aldy Hernandez  <aldyh@redhat.com>

	PR c++/55513
	* semantics.c (cxx_eval_builtin_function_call): Set non_constant_p
	after folding.

--- gcc-4.7.2/gcc/cp/semantics.c.~1~	2012-09-13 17:15:06.000000000 +0200
+++ gcc-4.7.2/gcc/cp/semantics.c	2012-12-09 11:52:18.000000000 +0100
@@ -46,6 +46,13 @@ along with GCC; see the file COPYING3.  
 #include "gimple.h"
 #include "bitmap.h"
 
+static bool verify_constant (tree, bool, bool *);
+#define VERIFY_CONSTANT(X)						\
+do {									\
+  if (verify_constant ((X), allow_non_constant, non_constant_p))	\
+    return t;								\
+ } while (0)
+
 /* There routines provide a modular interface to perform many parsing
    operations.  They may therefore be used during actual parsing, or
    during template instantiation, which may be regarded as a
@@ -6383,7 +6390,9 @@ cxx_eval_builtin_function_call (const co
     return t;
   new_call = build_call_array_loc (EXPR_LOCATION (t), TREE_TYPE (t),
                                    CALL_EXPR_FN (t), nargs, args);
-  return fold (new_call);
+  new_call = fold (new_call);
+  VERIFY_CONSTANT (new_call);
+  return new_call;
 }
 
 /* TEMP is the constant value of a temporary object of type TYPE.  Adjust
@@ -6687,11 +6696,6 @@ verify_constant (tree t, bool allow_non_
     }
   return *non_constant_p;
 }
-#define VERIFY_CONSTANT(X)						\
-do {									\
-  if (verify_constant ((X), allow_non_constant, non_constant_p))	\
-    return t;								\
- } while (0)
 
 /* Subroutine of cxx_eval_constant_expression.
    Attempt to reduce the unary expression tree T to a compile time value.
--- gcc-4.7.2/gcc/testsuite/g++.dg/pr55513.C.~1~	1970-01-01 01:00:00.000000000 +0100
+++ gcc-4.7.2/gcc/testsuite/g++.dg/pr55513.C	2012-12-09 12:01:31.000000000 +0100
@@ -0,0 +1,12 @@
+// { dg-do compile }
+// { dg-options "-O0 -fdump-tree-gimple" }
+
+main ()
+{
+  char s[10];
+  const int t = (__builtin_memcpy (s, "Hello", 6), 777);
+  __builtin_printf ("%d %s\n", t, s);
+}
+
+// { dg-final { scan-tree-dump-times "memcpy" 1 "gimple" } }
+// { dg-final { cleanup-tree-dump "gimple" } }