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[C++ PATCH] Fix up __builtin_is_constant_evaluated handling in array type sizes (PR c++/88446)


Hi!

As mentioned in the PR, while we allow VLAs in some contexts in C++ as
an extension, they aren't standard and the standard requires in those spots
constant expressions, thus __builtin_is_constant_evaluated () needs to be
true if those sizes are indeed constant expressions.

Fixed by calling cxx_eval_outermost_constant_expr with
pretend_const_required too in that case.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2018-12-11  Jakub Jelinek  <jakub@redhat.com>

	PR c++/88446
	* cp-tree.h (maybe_constant_value): Add pretend_const_required
	argument.
	* constexpr.c (maybe_constant_value): Likewise.  If true, don't
	cache and call cxx_eval_outermost_constant_expr with true as
	pretend_const_required.
	* decl.c (compute_array_index_type_loc): Call maybe_constant_value
	with true as pretend_const_required.

	* g++.dg/cpp2a/is-constant-evaluated3.C: New test.

--- gcc/cp/cp-tree.h.jj	2018-12-07 00:23:15.024998595 +0100
+++ gcc/cp/cp-tree.h	2018-12-11 13:55:51.933845503 +0100
@@ -7663,7 +7663,7 @@ extern bool require_rvalue_constant_expr
 extern bool require_potential_rvalue_constant_expression (tree);
 extern tree cxx_constant_value			(tree, tree = NULL_TREE);
 extern tree cxx_constant_init			(tree, tree = NULL_TREE);
-extern tree maybe_constant_value		(tree, tree = NULL_TREE);
+extern tree maybe_constant_value		(tree, tree = NULL_TREE, bool = false);
 extern tree maybe_constant_init			(tree, tree = NULL_TREE, bool = false);
 extern tree fold_non_dependent_expr		(tree, tsubst_flags_t = tf_warning_or_error);
 extern tree fold_simple				(tree);
--- gcc/cp/constexpr.c.jj	2018-12-11 12:01:27.968941683 +0100
+++ gcc/cp/constexpr.c	2018-12-11 13:56:50.382890876 +0100
@@ -5244,7 +5244,7 @@ fold_simple (tree t)
 static GTY((deletable)) hash_map<tree, tree> *cv_cache;
 
 tree
-maybe_constant_value (tree t, tree decl)
+maybe_constant_value (tree t, tree decl, bool pretend_const_required)
 {
   tree r;
 
@@ -5261,6 +5261,9 @@ maybe_constant_value (tree t, tree decl)
     /* No caching or evaluation needed.  */
     return t;
 
+  if (pretend_const_required)
+    return cxx_eval_outermost_constant_expr (t, true, true, true, decl);
+
   if (cv_cache == NULL)
     cv_cache = hash_map<tree, tree>::create_ggc (101);
   if (tree *cached = cv_cache->get (t))
--- gcc/cp/decl.c.jj	2018-12-07 00:23:15.000000000 +0100
+++ gcc/cp/decl.c	2018-12-11 13:57:30.779231098 +0100
@@ -9645,7 +9645,10 @@ compute_array_index_type_loc (location_t
 	{
 	  size = instantiate_non_dependent_expr_sfinae (size, complain);
 	  size = build_converted_constant_expr (size_type_node, size, complain);
-	  size = maybe_constant_value (size);
+	  /* Pedantically a constant expression is required here and so
+	     __builtin_is_constant_evaluated () should fold to true if it
+	     is successfully folded into a constant.  */
+	  size = maybe_constant_value (size, NULL_TREE, true);
 
 	  if (!TREE_CONSTANT (size))
 	    size = osize;
--- gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated3.C.jj	2018-12-11 14:11:34.235458615 +0100
+++ gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated3.C	2018-12-11 14:09:55.319073715 +0100
@@ -0,0 +1,26 @@
+// P0595R1
+// { dg-do run { target c++14 } }
+
+struct false_type { static constexpr bool value = false; };
+struct true_type { static constexpr bool value = true; };
+template<class T, class U>
+struct is_same : false_type {};
+template<class T>
+struct is_same<T, T> : true_type {};
+
+int a[__builtin_is_constant_evaluated () ? 1 : 2];
+int b[1];
+static_assert (is_same<decltype (a), decltype (b)>::value, "");
+
+int
+main ()
+{
+  int c[__builtin_is_constant_evaluated () ? 3 : 4];
+  int d[3];
+  static_assert (is_same<decltype (c), decltype (d)>::value, "");
+  int (*e)[7][9] = new int[__builtin_is_constant_evaluated () ? -1 : 5]
+			  [__builtin_is_constant_evaluated () ? 7 : 8]
+			  [__builtin_is_constant_evaluated () ? 9 : 10];
+  e[0][0][0] = 6;
+  delete[] e;
+}

	Jakub


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