c, tree: Fix ICE from get_parm_array_spec [PR97860]

The C and C++ FEs handle zero sized arrays differently, C uses
NULL TYPE_MAX_VALUE on non-NULL TYPE_DOMAIN on complete ARRAY_TYPEs
with bitsize_zero_node TYPE_SIZE, while C++ FE likes to set
TYPE_MAX_VALUE to the largest value (and min to the lowest).

Martin has used array_type_nelts in get_parm_array_spec where the
function on the C form of [0] arrays returns error_mark_node and the code
crashes soon afterwards.  The following patch teaches array_type_nelts about
this (e.g. dwarf2out already handles that as [0]).  While it will change
what is_empty_type returns for certain types (e.g. struct S { int a[0]; };),
as those types occupy zero bits in C, it should make an ABI difference.

So, the tree.c change makes the c-decl.c code handle the [0] arrays
like any other constant extents, and the c-decl.c change just makes sure
that if we'd run into error_mark_node e.g. from the VLA expressions, we
don't crash on those.

2020-11-19  Jakub Jelinek  <jakub@redhat.com>

	PR c/97860
	* tree.c (array_type_nelts): For complete arrays with zero min
	and NULL max and zero size return -1.

	* c-decl.c (get_parm_array_spec): Bail out of nelts is
	error_operand_p.

	* gcc.dg/pr97860.c: New test.

1 files changed, 11 insertions, 1 deletions

diff --git a/gcc/tree.c b/gcc/tree.c
index 0043855..531fe96 100644
--- a/gcc/tree.c
+++ b/gcc/tree.c
@@ -3483,7 +3483,17 @@ array_type_nelts (const_tree type)
 
   /* TYPE_MAX_VALUE may not be set if the array has unknown length.  */
   if (!max)
-    return error_mark_node;
+    {
+      /* zero sized arrays are represented from C FE as complete types with
+	 NULL TYPE_MAX_VALUE and zero TYPE_SIZE, while C++ FE represents
+	 them as min 0, max -1.  */
+      if (COMPLETE_TYPE_P (type)
+	  && integer_zerop (TYPE_SIZE (type))
+	  && integer_zerop (min))
+	return build_int_cst (TREE_TYPE (min), -1);
+
+      return error_mark_node;
+    }
 
   return (integer_zerop (min)
 	  ? max