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

[Jason Merrill <jason@redhat.com>]

On 6/24/23 09:24, Nathaniel Shead wrote:
> On Fri, Jun 23, 2023 at 11:59:51AM -0400, Patrick Palka wrote:
>> 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.
> 
> Thanks for the review! Fixed.
> 
>>>
>>> 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?
> 
> Ah thanks, I didn't notice this; this simplifies the change a fair bit.
> How about this patch instead?
> 
> Regtested on x86_64-pc-linux-gnu.
> 
> -- 8< --
> 
> This patch ensures that checks for usages of 'auto' in function
> parameters also consider parameter packs, since 'type_uses_auto' does
> not seem to consider this case.
> 
> 	PR c++/103497
> 
> gcc/cp/ChangeLog:
> 
> 	* decl.cc (grokdeclarator): Check for decltype(auto) in
> 	parameter pack.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/decltype-auto-103497.C: New test.
> 
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
>   gcc/cp/decl.cc                                    | 3 +++
>   gcc/testsuite/g++.dg/cpp1y/decltype-auto-103497.C | 8 ++++++++
>   2 files changed, 11 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/decltype-auto-103497.C
> 
> diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> index 60f107d50c4..aaf691fce68 100644
> --- a/gcc/cp/decl.cc
> +++ b/gcc/cp/decl.cc
> @@ -14044,6 +14044,9 @@ grokdeclarator (const cp_declarator *declarator,
>   	error ("cannot use %<::%> in parameter declaration");
>   
>         tree auto_node = type_uses_auto (type);
> +      if (!auto_node && parameter_pack_p)
> +	auto_node = type_uses_auto (PACK_EXPANSION_PATTERN (type));

Hmm, I wonder if type_uses_auto should look into PACK_EXPANSION_PATTERN 
itself.  Would that break anything?

> +
>         if (auto_node && !(cxx_dialect >= cxx17 && template_parm_flag))
>   	{
>   	  if (cxx_dialect >= cxx14)
> diff --git a/gcc/testsuite/g++.dg/cpp1y/decltype-auto-103497.C b/gcc/testsuite/g++.dg/cpp1y/decltype-auto-103497.C
> new file mode 100644
> index 00000000000..cedd661710c
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/decltype-auto-103497.C
> @@ -0,0 +1,8 @@
> +// PR c++/103497
> +// { dg-do compile { target c++14 } }
> +
> +void foo(decltype(auto)... args);  // { dg-error "cannot declare a parameter with .decltype.auto.." }
> +
> +int main() {
> +  foo();
> +}