[PATCH v2] c++: Fix C++11 attribute propagation [PR106712]

[Marek Polacek <polacek@redhat.com>]

On Mon, Aug 29, 2022 at 01:32:29PM -0400, Jason Merrill wrote:
> On 8/26/22 19:01, Marek Polacek wrote:
> > When we have
> > 
> >    [[noreturn]] int fn1 [[nodiscard]](), fn2();
> > 
> > "noreturn" should apply to both fn1 and fn2 but "nodiscard" only to fn1:
> > [dcl.pre]/3: "The attribute-specifier-seq appertains to each of
> > the entities declared by the declarators of the init-declarator-list."
> > [dcl.spec.general]: "The attribute-specifier-seq affects the type
> > only for the declaration it appears in, not other declarations involving
> > the same type."
> > 
> > As Ed Catmur correctly analyzed, this is because, for the test above,
> > we call start_decl with prefix_attributes=noreturn, but this line:
> > 
> >    attributes = attr_chainon (attributes, prefix_attributes);
> > 
> > results in attributes == prefix_attributes, because chainon sees
> > that attributes is null so it just returns prefix_attributes.  Then
> > in grokdeclarator we reach
> > 
> >    *attrlist = attr_chainon (*attrlist, declarator->std_attributes);
> > 
> > which modifies prefix_attributes so now it's "noreturn, nodiscard"
> > and so fn2 is wrongly marked nodiscard as well.  Fixed by copying
> > prefix_attributes.
> 
> How about reversing the order of arguments to the call in grokdeclarator, so
> that the prefix attributes can remain shared at the end of the list?

Thanks, that seems like a cheaper solution.  It works because this way
we tack the prefix attributes onto ->std_attributes, avoiding modifying
prefix_attributes.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
When we have

  [[noreturn]] int fn1 [[nodiscard]](), fn2();

"noreturn" should apply to both fn1 and fn2 but "nodiscard" only to fn1:
[dcl.pre]/3: "The attribute-specifier-seq appertains to each of
the entities declared by the declarators of the init-declarator-list."
[dcl.spec.general]: "The attribute-specifier-seq affects the type
only for the declaration it appears in, not other declarations involving
the same type."

As Ed Catmur correctly analyzed, this is because, for the test above,
we call start_decl with prefix_attributes=noreturn, but this line:

  attributes = attr_chainon (attributes, prefix_attributes);

results in attributes == prefix_attributes, because chainon sees
that attributes is null so it just returns prefix_attributes.  Then
in grokdeclarator we reach

  *attrlist = attr_chainon (*attrlist, declarator->std_attributes);

which modifies prefix_attributes so now it's "noreturn, nodiscard"
and so fn2 is wrongly marked nodiscard as well.  Fixed by reversing
the order of arguments to attr_chainon.  That way, we tack the prefix
attributes onto ->std_attributes, avoiding modifying prefix_attributes.

	PR c++/106712

gcc/cp/ChangeLog:

	* decl.cc (grokdeclarator): Reverse the order of arguments to
	attr_chainon.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/gen-attrs-77.C: New test.
---
 gcc/cp/decl.cc                            |  2 +-
 gcc/testsuite/g++.dg/cpp0x/gen-attrs-77.C | 17 +++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/gen-attrs-77.C

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index d46a347a6c7..b72b2a8456b 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -13474,7 +13474,7 @@ grokdeclarator (const cp_declarator *declarator,
       /* [dcl.meaning]/1: The optional attribute-specifier-seq following
 	 a declarator-id appertains to the entity that is declared.  */
       if (declarator->std_attributes != error_mark_node)
-	*attrlist = attr_chainon (*attrlist, declarator->std_attributes);
+	*attrlist = attr_chainon (declarator->std_attributes, *attrlist);
       else
 	/* We should have already diagnosed the issue (c++/78344).  */
 	gcc_assert (seen_error ());
diff --git a/gcc/testsuite/g++.dg/cpp0x/gen-attrs-77.C b/gcc/testsuite/g++.dg/cpp0x/gen-attrs-77.C
new file mode 100644
index 00000000000..2c41c62f33b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/gen-attrs-77.C
@@ -0,0 +1,17 @@
+// PR c++/106712
+// { dg-do compile { target c++11 } }
+
+[[noreturn]] int f1 [[nodiscard]](), f2 ();
+[[nodiscard]] int f3 (), f4 ();
+int f5 [[nodiscard]](), f6 ();
+
+int
+main ()
+{
+  f1 (); // { dg-warning "ignoring" }
+  f2 ();
+  f3 (); // { dg-warning "ignoring" }
+  f4 (); // { dg-warning "ignoring" }
+  f5 (); // { dg-warning "ignoring" }
+  f6 ();
+}

base-commit: 60d1d296b42b22b08d4eaa38bea02100c07261ac
-- 
2.37.2