c++: modules and function attributes

30_threads/stop_token/stop_source/109339.cc was failing because we weren't
representing attribute access on the METHOD_TYPE for _Stop_state_ref.

The modules code expected attributes to appear on tt_variant_type and not
on tt_derived_type, but that's backwards since build_type_attribute_variant
gives a type with attributes its own TYPE_MAIN_VARIANT.

gcc/cp/ChangeLog:

	* module.cc (trees_out::type_node): Write attributes for
	tt_derived_type, not tt_variant_type.
	(trees_in::tree_node): Likewise for reading.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/attrib-2_a.C: New test.
	* g++.dg/modules/attrib-2_b.C: New test.

gcc/cp/module.cc

@@ -9189,7 +9189,10 @@ trees_out::type_node (tree type)
 	  tree_node (raises);
 	}
-      tree_node (TYPE_ATTRIBUTES (type));
+      /* build_type_attribute_variant creates a new TYPE_MAIN_VARIANT, so
+	 variants should all have the same set of attributes.  */
+      gcc_checking_assert (TYPE_ATTRIBUTES (type)
+			   == TYPE_ATTRIBUTES (TYPE_MAIN_VARIANT (type)));
       if (streaming_p ())
 	{
@@ -9406,6 +9409,8 @@ trees_out::type_node (tree type)
       break;
     }
+  tree_node (TYPE_ATTRIBUTES (type));
+
   /* We may have met the type during emitting the above.  */
   if (ref_node (type) != WK_none)
     {
@@ -10090,6 +10095,13 @@ trees_in::tree_node (bool is_use)
 	    break;
 	  }
+	/* In the exporting TU, a derived type with attributes was built by
+	   build_type_attribute_variant as a distinct copy, with itself as
+	   TYPE_MAIN_VARIANT.  We repeat that on import to get the version
+	   without attributes as TYPE_CANONICAL.  */
+	if (tree attribs = tree_node ())
+	  res = cp_build_type_attribute_variant (res, attribs);
+
 	int tag = i ();
 	if (!tag)
 	  {
@@ -10133,9 +10145,6 @@ trees_in::tree_node (bool is_use)
 	    TYPE_USER_ALIGN (res) = true;
 	  }
-	if (tree attribs = tree_node ())
-	  res = cp_build_type_attribute_variant (res, attribs);
-
 	int quals = i ();
 	if (quals >= 0 && !get_overrun ())
 	  res = cp_build_qualified_type (res, quals);

gcc/testsuite/g++.dg/modules/attrib-2_a.C

+// { dg-additional-options "-fmodules -Wno-global-module" }
+// { dg-module-cmi M }
+
+export module M;
+
+export
+{
+  struct A { int i; };
+
+  __attribute ((access (none, 1)))
+  void f(const A&);
+}

gcc/testsuite/g++.dg/modules/attrib-2_b.C

+// { dg-additional-options "-fmodules -Wmaybe-uninitialized" }
+
+import M;
+
+int main()
+{
+  A a;
+  f(a);
+}