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

[Nathaniel Shead <nathanieloshead@gmail.com>]

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.

-- 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();
+}
-- 
2.41.0