c++: make manifestly_const_eval tri-state

This patch converts the constexpr machinery's manifestly_const_eval flag
into a tri-state enum to allow us to express wanting to fold
__builtin_is_constant_evaluated to false via speculative constexpr
evaluation.  For now, only the maybe_constant_value entry point is
changed to take this enum; the others continue to take bool.  The
subsequent patch will teach cp_fold (which uses maybe_constant_value) to
fold the builtin to false when called from cp_fold_function and
cp_fully_fold_init.

gcc/cp/ChangeLog:

	* constexpr.cc (constexpr_call::manifestly_const_eval): Give
	it type int instead of bool.
	(constexpr_ctx::manifestly_const_eval): Give it type mce_value
	instead of bool.
	(cxx_eval_builtin_function_call): Adjust after making
	manifestly_const_eval tri-state.
	(cxx_eval_call_expression): Likewise.
	(cxx_eval_binary_expression): Likewise.
	(cxx_eval_conditional_expression): Likewise.
	(cxx_eval_constant_expression): Likewise.
	(cxx_eval_outermost_constant_expr): Likewise.
	(cxx_constant_value): Likewise.
	(cxx_constant_dtor): Likewise.
	(maybe_constant_value): Give manifestly_const_eval parameter
	type mce_value instead of bool and adjust accordingly.
	(fold_non_dependent_expr_template): Adjust call
	to cxx_eval_outermost_constant_expr.
	(fold_non_dependent_expr): Likewise.
	(maybe_constant_init_1): Likewise.
	* constraint.cc (satisfy_atom): Adjust call to
	maybe_constant_value.
	* cp-tree.h (enum class mce_value): Define.
	(maybe_constant_value): Adjust manifestly_const_eval parameter
	type and default argument.
	* decl.cc (compute_array_index_type_loc): Adjust call to
	maybe_constant_value.
	* pt.cc (convert_nontype_argument): Likewise.

1 files changed, 19 insertions, 1 deletions

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 4ecd9c7..5595335 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8459,6 +8459,24 @@ struct GTY((for_user)) constexpr_fundef {
   tree result;
 };
 
+/* Whether the current context is manifestly constant-evaluated.
+   Used by the constexpr machinery to control folding of
+   __builtin_is_constant_evaluated.  */
+
+enum class mce_value
+{
+  /* Unknown, so treat __builtin_is_constant_evaluated as non-constant.  */
+  mce_unknown = 0,
+  /* Fold it to true.  */
+  mce_true = 1,
+  /* Fold it to false.  Primarily used during cp_fold_function and
+     cp_fully_fold_init.  */
+  mce_false = -1,
+};
+constexpr mce_value mce_unknown = mce_value::mce_unknown;
+constexpr mce_value mce_true = mce_value::mce_true;
+constexpr mce_value mce_false = mce_value::mce_false;
+
 extern void fini_constexpr			(void);
 extern bool literal_type_p                      (tree);
 extern void maybe_save_constexpr_fundef		(tree);
@@ -8487,7 +8505,7 @@ inline tree cxx_constant_value (tree t, tsubst_flags_t complain)
 { return cxx_constant_value (t, NULL_TREE, complain); }
 extern void cxx_constant_dtor			(tree, tree);
 extern tree cxx_constant_init			(tree, tree = NULL_TREE);
-extern tree maybe_constant_value		(tree, tree = NULL_TREE, bool = false);
+extern tree maybe_constant_value		(tree, tree = NULL_TREE, mce_value = mce_unknown);
 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,