[PATCH] c++: Report invalid id-expression in decltype [PR100482]

[Nathaniel Shead <nathanieloshead@gmail.com>]

(Another) ping.

On Sun, Apr 30, 2023 at 12:00:05PM +1000, Nathaniel Shead wrote:
> This patch ensures that any errors raised by finish_id_expression when
> parsing a decltype expression are properly reported, rather than
> potentially going ignored and causing invalid code to be accepted.
> 
> We can also now remove the separate check for templates without args as
> this is also checked for in finish_id_expression.
> 
> 	PR 100482
> 
> gcc/cp/ChangeLog:
> 
> 	* parser.cc (cp_parser_decltype_expr): Report errors raised by
> 	finish_id_expression.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/pr100482.C: New test.
> 
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
>  gcc/cp/parser.cc                | 22 +++++++++++-----------
>  gcc/testsuite/g++.dg/pr100482.C | 11 +++++++++++
>  2 files changed, 22 insertions(+), 11 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/pr100482.C
> 
> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> index e5f032f2330..20ebcdc3cfd 100644
> --- a/gcc/cp/parser.cc
> +++ b/gcc/cp/parser.cc
> @@ -16508,10 +16508,6 @@ cp_parser_decltype_expr (cp_parser *parser,
>  	expr = cp_parser_lookup_name_simple (parser, expr,
>  					     id_expr_start_token->location);
>  
> -      if (expr && TREE_CODE (expr) == TEMPLATE_DECL)
> -	/* A template without args is not a complete id-expression.  */
> -	expr = error_mark_node;
> -
>        if (expr
>            && expr != error_mark_node
>            && TREE_CODE (expr) != TYPE_DECL
> @@ -16532,13 +16528,17 @@ cp_parser_decltype_expr (cp_parser *parser,
>                     &error_msg,
>  		   id_expr_start_token->location));
>  
> -          if (expr == error_mark_node)
> -            /* We found an id-expression, but it was something that we
> -               should not have found. This is an error, not something
> -               we can recover from, so note that we found an
> -               id-expression and we'll recover as gracefully as
> -               possible.  */
> -            id_expression_or_member_access_p = true;
> +	  if (error_msg)
> +	    {
> +	      /* We found an id-expression, but it was something that we
> +		 should not have found. This is an error, not something
> +		 we can recover from, so report the error we found and
> +		 we'll recover as gracefully as possible.  */
> +	      cp_parser_parse_definitely (parser);
> +	      cp_parser_error (parser, error_msg);
> +	      id_expression_or_member_access_p = true;
> +	      return error_mark_node;
> +	    }
>          }
>  
>        if (expr
> diff --git a/gcc/testsuite/g++.dg/pr100482.C b/gcc/testsuite/g++.dg/pr100482.C
> new file mode 100644
> index 00000000000..dcf6722fda5
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/pr100482.C
> @@ -0,0 +1,11 @@
> +// { dg-do compile { target c++11 } }
> +
> +namespace N {}
> +decltype(std) x;   // { dg-error "expected primary-expression" }
> +
> +struct S {};
> +decltype(S) y;  // { dg-error "argument to .decltype. must be an expression" }
> +
> +template <typename T>
> +struct U {};
> +decltype(U) z;  // { dg-error "missing template arguments" }
> -- 
> 2.40.0
>