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

[Jason Merrill <jason@redhat.com>]

On 6/30/23 03:05, Nathaniel Shead wrote:
> On Thu, Jun 29, 2023 at 01:43:07PM -0400, Jason Merrill wrote:
>> 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?
> 
> I gave that a try and it seems to work fine.
> 
> Regtested on x86_64-pc-linux-gnu.

Pushed, thanks.

> -- 8< --
> 
> This patch ensures 'type_uses_auto' also checks for usages of 'auto' in
> parameter packs.
> 
> 	PR c++/103497
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.cc (type_uses_auto): Check inside parameter packs.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/decltype-auto-103497.C: New test.
> 
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
>   gcc/cp/pt.cc                                      | 7 ++++++-
>   gcc/testsuite/g++.dg/cpp1y/decltype-auto-103497.C | 8 ++++++++
>   2 files changed, 14 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/decltype-auto-103497.C
> 
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 2345a18becc..2a3ba5ab67d 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -31083,7 +31083,12 @@ type_uses_auto (tree type)
>   {
>     if (type == NULL_TREE)
>       return NULL_TREE;
> -  else if (flag_concepts_ts)
> +
> +  /* For parameter packs, check the contents of the pack.  */
> +  if (PACK_EXPANSION_P (type))
> +    type = PACK_EXPANSION_PATTERN (type);
> +
> +  if (flag_concepts_ts)
>       {
>         /* The Concepts TS allows multiple autos in one type-specifier; just
>   	 return the first one we find, do_auto_deduction will collect all of
> 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();
> +}