Re: [PATCH] c++: Fix ICE with parameter pack of decltype(auto) [PR103497]

[Patrick Palka <ppalka@redhat.com>]

Hi,

On Sat, 22 Apr 2023, Nathaniel Shead via Gcc-patches wrote:

> Bootstrapped and tested on x86_64-pc-linux-gnu.
> 
> -- 8< --
> 
> This patch raises an error early when the decltype(auto) specifier is
> used as a parameter of a function. This prevents any issues with an
> unexpected tree type later on when performing the call.

Thanks very much for the patch!  Some minor comments below.

> 
> 	PR 103497

We should include the bug component name when referring to the PR in the
commit message (i.e. PR c++/103497) so that upon pushing the patch the
post-commit hook automatically adds a comment to the PR reffering to the
commit.  I could be wrong but AFAIK the hook only performs this when the
component name is included.

> 
> gcc/cp/ChangeLog:
> 
> 	* parser.cc (cp_parser_simple_type_specifier): Add check for
> 	decltype(auto) as function parameter.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/pr103497.C: New test.
> 
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
>  gcc/cp/parser.cc                | 10 ++++++++++
>  gcc/testsuite/g++.dg/pr103497.C |  7 +++++++
>  2 files changed, 17 insertions(+)
>  create mode 100644 gcc/testsuite/g++.dg/pr103497.C
> 
> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> index e5f032f2330..1415e07e152 100644
> --- a/gcc/cp/parser.cc
> +++ b/gcc/cp/parser.cc
> @@ -19884,6 +19884,16 @@ cp_parser_simple_type_specifier (cp_parser* parser,
>        && cp_lexer_peek_nth_token (parser->lexer, 2)->type != CPP_SCOPE)
>      {
>        type = saved_checks_value (token->u.tree_check_value);
> +      /* Within a function parameter declaration, decltype(auto) is always an
> +	 error.  */
> +      if (parser->auto_is_implicit_function_template_parm_p
> +	  && TREE_CODE (type) == TEMPLATE_TYPE_PARM

We could check is_auto (type) here instead, to avoid any confusion with
checking AUTO_IS_DECLTYPE for a non-auto TEMPLATE_TYPE_PARM.

> +	  && AUTO_IS_DECLTYPE (type))
> +	{
> +	  error_at (token->location,
> +		    "cannot declare a parameter with %<decltype(auto)%>");
> +	  type = error_mark_node;
> +	}
>        if (decl_specs)
>  	{
>  	  cp_parser_set_decl_spec_type (decl_specs, type,
> diff --git a/gcc/testsuite/g++.dg/pr103497.C b/gcc/testsuite/g++.dg/pr103497.C
> new file mode 100644
> index 00000000000..bcd421c2907
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/pr103497.C
> @@ -0,0 +1,7 @@
> +// { dg-do compile { target c++14 } }
> +
> +void foo(decltype(auto)... args);  // { dg-error "parameter with .decltype.auto..|no parameter packs" }

I noticed for

  void foo(decltype(auto) arg);

we already issue an identical error from grokdeclarator.  Perhaps we could
instead extend the error handling there to detect decltype(auto)... as well,
rather than adding new error handling in cp_parser_simple_type_specifier?

> +
> +int main() {
> +  foo();
> +}
> -- 
> 2.34.1
> 
>