c++: format attribute redeclaration [PR116954]

Here when merging the two decls, remove_contract_attributes loses
ATTR_IS_DEPENDENT on the format attribute, so apply_late_template_attributes
just returns, so the attribute doesn't get propagated to the type where the
warning looks for it.

Fixed by using copy_node instead of tree_cons to preserve flags.

	PR c++/116954

gcc/cp/ChangeLog:

	* contracts.cc (remove_contract_attributes): Preserve flags
	on the attribute list.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wformat-3.C: New test.

2 files changed, 24 insertions, 1 deletions

diff --git a/gcc/cp/contracts.cc b/gcc/cp/contracts.cc
index f2b126c..3ca2102 100644
--- a/gcc/cp/contracts.cc
+++ b/gcc/cp/contracts.cc
@@ -863,7 +863,11 @@ remove_contract_attributes (tree fndecl)
   tree list = NULL_TREE;
   for (tree p = DECL_ATTRIBUTES (fndecl); p; p = TREE_CHAIN (p))
     if (!cxx_contract_attribute_p (p))
-      list = tree_cons (TREE_PURPOSE (p), TREE_VALUE (p), list);
+      {
+	tree nl = copy_node (p);
+	TREE_CHAIN (nl) = list;
+	list = nl;
+      }
   DECL_ATTRIBUTES (fndecl) = nreverse (list);
 }
 
diff --git a/gcc/testsuite/g++.dg/warn/Wformat-3.C b/gcc/testsuite/g++.dg/warn/Wformat-3.C
new file mode 100644
index 0000000..e308530
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wformat-3.C
@@ -0,0 +1,19 @@
+// PR c++/116954
+// { dg-additional-options -Wformat }
+
+#ifndef WORKS
+template<int N>
+int fn(char (&buf)[N], const char fmt[], ...)
+  __attribute__ ((__format__ (__printf__, 2, 3)));
+#endif
+
+template<int  N>
+__attribute__ ((__format__ (__printf__, 2, 3)))
+int fn(char (&)[N], const char [], ...)
+{ return 0; }
+
+int main()
+{
+  char buf[20];
+  return fn(buf, "%s", 42); /* { dg-warning "Wformat" } */
+}