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

[Marek Polacek <polacek@redhat.com>]

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.

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

	PR c++/106712

gcc/cp/ChangeLog:

	* decl.cc (start_decl): Copy prefix_attributes.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/gen-attrs-77.C: New test.
---
 gcc/cp/decl.cc                            |  3 ++-
 gcc/testsuite/g++.dg/cpp0x/gen-attrs-77.C | 17 +++++++++++++++++
 2 files changed, 19 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..9fa80b926d5 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -5566,7 +5566,8 @@ start_decl (const cp_declarator *declarator,
   *pushed_scope_p = NULL_TREE;
 
   if (prefix_attributes != error_mark_node)
-    attributes = attr_chainon (attributes, prefix_attributes);
+    /* Don't let grokdeclarator modify prefix_attributes.  */
+    attributes = attr_chainon (attributes, copy_list (prefix_attributes));
 
   decl = grokdeclarator (declarator, declspecs, NORMAL, initialized,
 			 &attributes);
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: 390f94eee1ae694901f896ac45bfb148f8126baa
-- 
2.37.2