c++: Fix guard variable and attribute weak.

My patch for PR 91476 worked for decls that are implicitly comdat/weak due
to C++ linkage rules, but broke variables explicitly marked weak.

	PR c++/93477
	PR c++/91476
	* decl2.c (copy_linkage): Do copy DECL_ONE_ONLY and DECL_WEAK.

gcc/cp/ChangeLog

 2020-01-28  Jason Merrill  <jason@redhat.com>
 
+	PR c++/93477
+	PR c++/91476
+	* decl2.c (copy_linkage): Do copy DECL_ONE_ONLY and DECL_WEAK.
+
 	PR c++/90546
 	* call.c (build_user_type_conversion_1): Allow a template conversion
 	returning an rvalue reference to bind directly to an lvalue.

gcc/cp/decl2.c

@@ -3228,8 +3228,12 @@ copy_linkage (tree guard, tree decl)
     {
       CP_DECL_THREAD_LOCAL_P (guard) = CP_DECL_THREAD_LOCAL_P (decl);
       set_decl_tls_model (guard, DECL_TLS_MODEL (decl));
-      /* We can't rely on DECL_WEAK (decl) or DECL_ONE_ONLY (decl) here, as
-	 they may not be set until import_export_decl at EOF.  */
+      if (DECL_ONE_ONLY (decl))
+	make_decl_one_only (guard, cxx_comdat_group (guard));
+      if (TREE_PUBLIC (decl))
+	DECL_WEAK (guard) = DECL_WEAK (decl);
+      /* Also check vague_linkage_p, as DECL_WEAK and DECL_ONE_ONLY might not
+	 be set until import_export_decl at EOF.  */
       if (vague_linkage_p (decl))
 	comdat_linkage (guard);
       DECL_VISIBILITY (guard) = DECL_VISIBILITY (decl);

gcc/testsuite/g++.dg/abi/guard4.C

+// PR c++/93477
+// { dg-require-weak }
+
+namespace x {
+  struct s {
+    s() {}
+    static int a;
+  };
+  // { dg-final { scan-assembler {.weak[^\n]*_ZGVN1x1bE} } }
+  struct s __attribute__((weak)) b = s();
+}