[PATCH] Fix PR68087

[Markus Trippelsdorf]

Hi,

the patch below fixes PR68087, an ICE caused by referring to a negative
constexpr array element.

Bootstrapped and tested on ppc64le.
OK for trunk?

gcc-5 also needs a slightly different fix. I'll post that as a
follow-up, once this one gets approved.

        PR c++/68087
        * constexpr.c (cxx_eval_array_reference): Guard call to
        tree_to_shwi().

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index ebca411b3eb4..0828a90b0e75 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1782,8 +1782,7 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree 
t,
       gcc_unreachable ();
     }
 
-  i = tree_to_shwi (index);
-  if (i < 0)
+  if (!tree_fits_shwi_p (index) || tree_to_shwi (index) < 0)
     {
       if (!ctx->quiet)
        error ("negative array subscript");
@@ -1792,6 +1791,7 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree 
t,
     }
 
   bool found;
+  i = tree_to_shwi (index);
   if (TREE_CODE (ary) == CONSTRUCTOR)
     {
       HOST_WIDE_INT ix = find_array_ctor_elt (ary, index);
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-array13.C 
b/gcc/testsuite/g++.dg/cpp0x/constexpr-array13.C
new file mode 100644
index 000000000000..ef18a60f3038
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-array13.C
@@ -0,0 +1,7 @@
+// PR c++/68087
+// { dg-do compile { target c++11 } }
+
+constexpr char c[] = "hello";
+constexpr const char *p = c;
+
+static_assert(*(p - 1) == 'h', ""); // { dg-error "non-constant|negative" }
-- 
Markus