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

On 12/11/18 3:35 PM, Marek Polacek wrote:
On Tue, Dec 11, 2018 at 05:39:25PM +0100, Jakub Jelinek wrote:

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  <>

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

Could you please describe the new param in the comment?

--- 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);

Perhaps use /*pretend_const_required=*/true?

Please. And the name of the parameter should change to match the other patches. OK with those changes.


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