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

[Patrick Palka <ppalka@redhat.com>]

On Sat, Jun 24, 2023 at 9:24 AM Nathaniel Shead
<nathanieloshead@gmail.com> 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?

LGTM! Though I can't approve the patch myself.

>
> 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));
> +
>        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();
> +}
> --
> 2.41.0
>