[Bug c++/68087] [5/6 Regression] ICE with constexpr in array with negative index

[trippels at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68087

--- Comment #2 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
Perhaps:

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index ebca411..0828a90 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);