Implement -Wimplicit-fallthrough (version 7)

[Jason Merrill]

On Mon, Aug 29, 2016 at 7:58 AM, Marek Polacek <polacek@redhat.com> wrote:
> --- gcc/gcc/cp/parser.c
> +++ gcc/gcc/cp/parser.c
> @@ -5135,6 +5135,31 @@ cp_parser_primary_expression (cp_parser *parser,
>         case RID_AT_SELECTOR:
>           return cp_parser_objc_expression (parser);
>
> +       case RID_ATTRIBUTE:
> +         {
> +           /* This might be __attribute__((fallthrough));.  */
> +           tree attr = cp_parser_gnu_attributes_opt (parser);
> +           if (attr != NULL_TREE
> +               && is_attribute_p ("fallthrough", get_attribute_name (attr)))
> +             {
> +               tree fn = build_call_expr_internal_loc (token->location,
> +                                                       IFN_FALLTHROUGH,
> +                                                       void_type_node, 0);
> +               return cp_expr (fn, token->location);
> +             }
> +           else if (cp_lexer_next_token_is (parser->lexer, CPP_SEMICOLON))
> +             {
> +               cp_parser_error (parser, "only attribute %<fallthrough%> "
> +                                "can be used");

Let's say "can be applied to a null statement".

> @@ -10585,15 +10610,26 @@ cp_parser_statement (cp_parser* parser, tree in_statement_expr,
>         }
>        /* Look for an expression-statement instead.  */
>        statement = cp_parser_expression_statement (parser, in_statement_expr);
> +
> +      /* Handle [[fallthrough]];.  */
> +      if (std_attrs != NULL_TREE
> +         && is_attribute_p ("fallthrough", get_attribute_name (std_attrs))
> +         && statement == NULL_TREE)
> +       {
> +         tree fn = build_call_expr_internal_loc (statement_location,
> +                                                 IFN_FALLTHROUGH,
> +                                                 void_type_node, 0);

Let's use the 'statement' variable rather than a new variable fn so
that we set the call's location below.  Let's also warn here about
[[fallthrough]] on non-null expression statements.

> +         finish_expr_stmt (fn);

Let's clear std_attrs here...

> +       }

>    /* Set the line number for the statement.  */
>    if (statement && STATEMENT_CODE_P (TREE_CODE (statement)))
>      SET_EXPR_LOCATION (statement, statement_location);
>
> -  /* Note that for now, we don't do anything with c++11 statements
> -     parsed at this level.  */
> -  if (std_attrs != NULL_TREE)
> +  /* Allow "[[fallthrough]];", but warn otherwise.  */
> +  if (std_attrs != NULL_TREE
> +      && !is_attribute_p ("fallthrough", get_attribute_name (std_attrs)))

...so we don't need to check for fallthrough here.

> @@ -24114,6 +24164,16 @@ cp_parser_std_attribute (cp_parser *parser)
>                      " use %<gnu::deprecated%>");
>           TREE_PURPOSE (TREE_PURPOSE (attribute)) = get_identifier ("gnu");
>         }
> +      /* C++17 fallthrough attribute is equivalent to GNU's.  */
> +      else if (cxx_dialect >= cxx11

There's no point checking this, here or for attribute deprecated; if
we're in this function, we must be in C++11 or above.  Let's drop the
check in both places.

Jason