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.

From-SVN: r239338

1 files changed, 4 insertions, 0 deletions

diff --git a/gcc/cp/parser.h b/gcc/cp/parser.h
index 2d3feb6..6a52b12 100644
--- a/gcc/cp/parser.h
+++ b/gcc/cp/parser.h
@@ -336,6 +336,10 @@ struct GTY(()) cp_parser {
      a local class.  */
   bool in_function_body;
 
+  /* TRUE if we are parsing a C++17 discarded statement (the non-taken branch
+     of an if constexpr).  */
+  bool in_discarded_stmt;
+
   /* Nonzero if we're processing a __transaction_atomic or
      __transaction_relaxed statement.  */
   unsigned char in_transaction;