Make sure we aren't trying to do a nested instantiation in template context.

	* pt.c (instantiate_decl): Make sure we aren't trying to do a nested
	instantiation in template context.

From-SVN: r260124

gcc/cp/ChangeLog

 2018-05-09  Jason Merrill  <jason@redhat.com>
 
+	* pt.c (instantiate_decl): Make sure we aren't trying to do a nested
+	instantiation in template context.
+
 	* class.c (vbase_has_user_provided_move_assign): Use
 	user_provided_p.
 

gcc/cp/pt.c

@@ -23886,6 +23886,7 @@ instantiate_decl (tree d, bool defer_ok, bool expl_inst_class_mem_p)
     push_to_top_level ();
   else
     {
+      gcc_assert (!processing_template_decl);
       push_function_context ();
       cp_unevaluated_operand = 0;
       c_inhibit_evaluation_warnings = 0;