c++: Fix ICE with invalid array bounds [PR93789]

r7-2111 introduced maybe_constant_value in cp_fully_fold.
maybe_constant_value uses cxx_eval_outermost_constant_expr, which
can clear TREE_CONSTANT:
6510   else if (non_constant_p && TREE_CONSTANT (r))
[...]
6529       TREE_CONSTANT (r) = false;

In this test the array size is '(long int) "h"'.  This used to be
TREE_CONSTANT but given the change above, the flag will be cleared
when we cp_fully_fold the array size in compute_array_index_type_loc.
That means we don't emit an error in the
10391   else if (TREE_CONSTANT (size)
block in the same function, and we go on.  Then we compute ITYPE
using cp_build_binary_op and use maybe_constant_value on it and
suddenly we have something that's TREE_CONSTANT again.  And then we
crash in reshape_init_array_1 in tree_to_uhwi, because what we have
doesn't fit in an unsigned HWI.

icc accepts this code, but since we used to reject it, I see no desire
to make this work, so don't use the folded result when we've lost
the TREE_CONSTANT flag while evaluating the size.

2020-02-26  Marek Polacek  <polacek@redhat.com>

	PR c++/93789 - ICE with invalid array bounds.
	* decl.c (compute_array_index_type_loc): Don't use the folded
	size when folding cleared TREE_CONSTANT.

	* g++.dg/ext/vla22.C: New test.

gcc/cp/ChangeLog

+2020-02-26  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93789 - ICE with invalid array bounds.
+	* decl.c (compute_array_index_type_loc): Don't use the folded
+	size when folding cleared TREE_CONSTANT.
+
 2020-02-26  Iain Sandoe  <iain@sandoe.co.uk>
 
 	* class.c (classtype_has_non_deleted_copy_ctor): New.

gcc/cp/decl.c

@@ -10338,9 +10338,14 @@ compute_array_index_type_loc (location_t name_loc, tree name, tree size,
 	    pedwarn (loc, OPT_Wpedantic,
 		     "size of array is not an integral constant-expression");
 	}
-      /* Use the folded result for VLAs, too; it will have resolved
-	 SIZEOF_EXPR.  */
-      size = folded;
+      if (TREE_CONSTANT (size) && !TREE_CONSTANT (folded))
+	/* We might have lost the TREE_CONSTANT flag e.g. when we are
+	   folding a conversion from a pointer to integral type.  In that
+	   case issue an error below and don't treat this as a VLA.  */;
+      else
+	/* Use the folded result for VLAs, too; it will have resolved
+	   SIZEOF_EXPR.  */
+	size = folded;
     }
 
   /* Normally, the array-bound will be a constant.  */

gcc/testsuite/ChangeLog

+2020-02-26  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93789 - ICE with invalid array bounds.
+	* g++.dg/ext/vla22.C: New test.
+
 2020-02-26  David Malcolm  <dmalcolm@redhat.com>
 
 	PR analyzer/93950

gcc/testsuite/g++.dg/ext/vla22.C

+// PR c++/93789 - ICE with invalid array bounds.
+// { dg-do compile }
+// { dg-options "" }
+
+void
+f ()
+{
+  const int tbl[(long) "h"] = { 12 }; // { dg-error "size of array .tbl. is not an integral constant-expression" }
+}