c++: ICE during aggr CTAD for member tmpl [PR105476]

Here we're crashing from maybe_aggr_guide ultimately because
processing_template_decl isn't set when partially instantiating the
guide's parameter list; this causes us to force completion of the
dependent type Visitor_functior<Fn>, which of course fails and results
in an unexpected error_mark_node (the instantation should always succeed).

	PR c++/105476

gcc/cp/ChangeLog:

	* pt.cc (maybe_aggr_guide): Set processing_template_decl when
	partially instantiating the guide's parameter list.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-aggr13.C: New test.
	* g++.dg/cpp2a/class-deduction-aggr13a.C: New test.

gcc/cp/pt.cc

@@ -29569,7 +29569,11 @@ maybe_aggr_guide (tree tmpl, tree init, vec<tree,va_gc> *args)
 	 PARMS, so that its template level is properly reduced and we don't get
 	 mismatches when deducing types using the guide with PARMS.  */
       if (member_template_p)
-	parms = tsubst (parms, DECL_TI_ARGS (tmpl), complain, init);
+	{
+	  ++processing_template_decl;
+	  parms = tsubst (parms, DECL_TI_ARGS (tmpl), complain, init);
+	  --processing_template_decl;
+	}
     }
   else if (TREE_CODE (init) == TREE_LIST)
     {

gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr13.C

@@ -0,0 +1,11 @@
+// PR c++/105476
+// { dg-do compile { target c++17 } }
+
+template<class> struct Visitor_functor;
+
+template<class> struct Events {
+  template<class... Fn> struct Visitor : Visitor_functor<Fn>::type_t... { };
+};
+
+using ev_t = Events<int>;
+ev_t::Visitor v = { {} }; // { dg-error "too many initializers" }

gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr13a.C

@@ -0,0 +1,18 @@
+// PR c++/105476
+// { dg-do compile { target c++20 } }
+// A valid version of class-deduction-aggr13.C.
+
+template<class> struct Visitor_functor;
+
+template<> struct Visitor_functor<int> {
+  using type_t = int;
+};
+
+template<class T> struct Events {
+  template<class Fn=T> struct Visitor {
+    Visitor_functor<Fn>::type_t t;
+  };
+};
+
+using ev_t = Events<int>;
+ev_t::Visitor v = { {} };