Attempt at fixing RT #118555, https://rt.perl.org/Ticket/Display.html?id=118555

RT118555.diff

diff --git a/src/Perl6/Metamodel/BOOTSTRAP.nqp b/src/Perl6/Metamodel/BOOTSTRAP.nqp
index 4b2a7f3..3f1fd5f 100644
--- a/src/Perl6/Metamodel/BOOTSTRAP.nqp
+++ b/src/Perl6/Metamodel/BOOTSTRAP.nqp
@@ -563,16 +563,25 @@ my class Binder {
 
         # Otherwise, go by sigil to pick the correct default type of value.
         else {
+            my $nom_type := nqp::getattr($param, Parameter, '$!nominal_type');
             if $flags +& $SIG_ELEM_ARRAY_SIGIL {
-                my $result := nqp::create(Array);
+                my $nom_subtype := $nom_type.of;
+                my $type := $nom_subtype =:= Mu
+                    ?? Array
+                    !! Array.HOW.parameterize(Array, $nom_subtype);
+                my $result := nqp::create($type);
                 nqp::bindattr($result, List, '$!flattens', nqp::p6bool(1));
                 $result
             }
             elsif $flags +& $SIG_ELEM_HASH_SIGIL {
-                nqp::create(Hash)
+                my $nom_subtype := $nom_type.of;
+                my $type := $nom_subtype =:= Mu
+                    ?? Hash
+                    !! Hash.HOW.parameterize(Hash, $nom_subtype);
+                nqp::create($type)
             }
             else {
-                nqp::getattr($param, Parameter, '$!nominal_type');
+                $nom_type
             }
         }
     }