c++, contracts: Use contract scope for each contract condition.

This just wraps each contract condition check in a new scope (as
required by P2900).  I've actually just let it happen for c++2a
contracts as well, since we are going to deprecate them (and it
apparently does not alter their behaviour).

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

Author: iains

gcc/cp/cp-tree.h

@@ -1955,6 +1955,8 @@ struct GTY(()) saved_scope {
   int x_processing_contract_condition;
   int x_processing_contract_postcondition;
   int suppress_location_wrappers;
+  BOOL_BITFIELD x_processing_postcondition : 1;
+  BOOL_BITFIELD x_should_constify_contract : 1;
   BOOL_BITFIELD x_processing_explicit_instantiation : 1;
   BOOL_BITFIELD need_pop_function_context : 1;
   BOOL_BITFIELD x_processing_omp_trait_property_expr : 1;
@@ -2039,6 +2041,9 @@ extern GTY(()) struct saved_scope *scope_chain;
 
 #define processing_contract_postcondition scope_chain->x_processing_contract_postcondition
 
+#define processing_postcondition scope_chain->x_processing_postcondition
+#define should_constify_contract scope_chain->x_should_constify_contract
+
 #define in_discarded_stmt scope_chain->discarded_stmt
 #define in_consteval_if_p scope_chain->consteval_if_p
 

gcc/cp/parser.cc

@@ -13237,9 +13237,13 @@ cp_parser_statement (cp_parser* parser, tree in_statement_expr,
 		current_class_ref = view_as_const (current_class_ref_copy);
 
 	      /* Parse the condition.  */
+	      begin_scope (sk_contract, current_function_decl);
 	      ++processing_contract_condition;
+	      processing_postcondition = false;
+	      should_constify_contract = should_constify;
 	      cp_expr condition = cp_parser_conditional_expression (parser);
 	      --processing_contract_condition;
+	      pop_bindings_and_leave_scope ();
 
 	      /* Revert (any) constification of the current class object.  */
 	      current_class_ref = current_class_ref_copy;
@@ -31524,16 +31528,20 @@ cp_parser_contract_attribute_spec (cp_parser *parser, tree attribute,
 
       /* Parse the condition, ensuring that parameters or the return variable
 	 aren't flagged for use outside the body of a function.  */
+      begin_scope (sk_contract, current_function_decl);
       ++processing_contract_condition;
       if (postcondition_p)
 	++processing_contract_postcondition;
+      processing_postcondition = postcondition_p;
+      should_constify_contract = should_constify;
       cp_expr condition = cp_parser_conditional_expression (parser);
       if (postcondition_p)
 	--processing_contract_postcondition;
+      --processing_contract_condition;
+      pop_bindings_and_leave_scope ();
       /* Revert (any) constification of the current class object.  */
       current_class_ref = current_class_ref_copy;
       flag_contracts_nonattr_noconst = old_flag_contracts_nonattr_noconst;
-      --processing_contract_condition;
 
 	/* For natural syntax, we eat the parens here. For the attribute
 	syntax, it will be done one level up, we just need to skip to it. */
@@ -31643,13 +31651,15 @@ void cp_parser_late_contract_condition (cp_parser *parser,
 
   /* Parse the condition, ensuring that parameters or the return variable
      aren't flagged for use outside the body of a function.  */
+  begin_scope (sk_contract, fn);
   ++processing_contract_condition;
   if (POSTCONDITION_P (contract))
     ++processing_contract_postcondition;
   condition = cp_parser_conditional_expression (parser);
   if (POSTCONDITION_P (contract))
     --processing_contract_postcondition;
   --processing_contract_condition;
+  pop_bindings_and_leave_scope ();
 
   if (cp_lexer_next_token_is_not (parser->lexer, CPP_EOF))
       error_at (input_location,