c++: Propagate attributes to clones in duplicate_decls [PR67453]

On the following testcase where the cdtor attributes aren't on the
in-class declaration but on an out-of-class definition, the cdtors
have their clones created from the in-class declaration, and later on
duplicate_decls updates attributes on the abstract cdtors, but nothing
propagates them to the clones.

2020-11-06  Jakub Jelinek  <jakub@redhat.com>

	PR c++/67453
	* decl.c (duplicate_decls): Propagate DECL_ATTRIBUTES and
	DECL_PRESERVE_P from olddecl to its clones if any.

	* g++.dg/ext/attr-used-2.C: New test.

gcc/cp/decl.c

@@ -2921,6 +2921,16 @@ duplicate_decls (tree newdecl, tree olddecl, bool hiding, bool was_hidden)
 	snode->remove ();
     }
+  if (TREE_CODE (olddecl) == FUNCTION_DECL)
+    {
+      tree clone;
+      FOR_EACH_CLONE (clone, olddecl)
+	{
+	  DECL_ATTRIBUTES (clone) = DECL_ATTRIBUTES (olddecl);
+	  DECL_PRESERVE_P (clone) |= DECL_PRESERVE_P (olddecl);
+	}
+    }
+
   /* Remove the associated constraints for newdecl, if any, before
      reclaiming memory. */
   if (flag_concepts)

gcc/testsuite/g++.dg/ext/attr-used-2.C

+// PR c++/67453
+// { dg-do compile }
+// { dg-final { scan-assembler "_ZN1SC\[12]Ev" } }
+// { dg-final { scan-assembler "_ZN1SD\[12]Ev" } }
+// { dg-final { scan-assembler "_ZN1SC\[12]ERKS_" } }
+
+struct S {
+    S();
+    ~S();
+    S(const S&);
+};
+
+__attribute__((used)) inline S::S()  { }
+__attribute__((used)) inline S::~S() { }
+__attribute__((used)) inline S::S(const S&) { }