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

[Nathaniel Shead <nathanieloshead@gmail.com>]

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));
+
       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