Implement C++17 constexpr if.

	* cp-tree.h (IF_STMT_CONSTEXPR_P): New.
	* name-lookup.c (push_to_top_level, pop_from_top_level_1): Handle it.
	* parser.h (struct cp_parser): Add in_discarded_stmt field.
	* parser.c (cp_parser_selection_statement): Handle 'if constexpr'.
	(cp_parser_jump_statement): Avoid deducing from a discarded return.
	* pt.c (tsubst_expr): Only instantiate taken branch of constexpr if.
	* semantics.c (begin_if_stmt): Set the binding level this_entity.
	(finish_if_stmt_cond): Require the condition of a
	constexpr if to be constant.
	* decl.c (level_for_constexpr_if): New.
	(named_label_entry): Add in_constexpr_if field.
	(poplevel_named_label_1): Set it.
	(check_goto): Check it.
	(check_previous_goto_1): Check level_for_constexpr_if.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@239338 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 9 insertions, 2 deletions

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index bffdddbb965..a2e04f6b61d 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -715,6 +715,7 @@ begin_if_stmt (void)
   scope = do_pushlevel (sk_cond);
   r = build_stmt (input_location, IF_STMT, NULL_TREE,
 		  NULL_TREE, NULL_TREE, scope);
+  current_binding_level->this_entity = r;
   begin_cond (&IF_COND (r));
   return r;
 }
@@ -722,12 +723,18 @@ begin_if_stmt (void)
 /* Process the COND of an if-statement, which may be given by
    IF_STMT.  */
 
-void
+tree
 finish_if_stmt_cond (tree cond, tree if_stmt)
 {
-  finish_cond (&IF_COND (if_stmt), maybe_convert_cond (cond));
+  cond = maybe_convert_cond (cond);
+  if (IF_STMT_CONSTEXPR_P (if_stmt)
+      && require_potential_rvalue_constant_expression (cond)
+      && !value_dependent_expression_p (cond))
+    cond = cxx_constant_value (cond, NULL_TREE);
+  finish_cond (&IF_COND (if_stmt), cond);
   add_stmt (if_stmt);
   THEN_CLAUSE (if_stmt) = push_stmt_list ();
+  return cond;
 }
 
 /* Finish the then-clause of an if-statement, which may be given by