Check for definiteness of optional @/% defaults too

If we're going so far as to avoid other compiler errors, one such error
could be to miss an instantiation. Reorganize these tests for clarity
while we're here.

S06-signature/defaults.t

@@ -10,7 +10,7 @@ Tests assigning default values to variables of type code in sub definitions.
 
 # L<S06/Optional parameters/Default values can be calculated at run-time>
 
-plan 13;
+plan 17;
 
 sub doubler($x) { return 2 * $x }
 
@@ -72,15 +72,23 @@ ok((MyPack::val_v), "default sub called in package namespace");
 
 # https://github.com/rakudo/rakudo/issues/4647
 {
-    -> :@l { cmp-ok @l.WHAT, &[=:=], Array,
-        'untyped optional arrays get their default' }();
-    -> :%h { cmp-ok %h.WHAT, &[=:=], Hash,
-        'untyped optional hashes get their default' }();
-
-    -> Str :@l { cmp-ok @l.WHAT, &[=:=], Array[Str],
-        'typed optional arrays get their default' }();
-    -> Str :%h { cmp-ok %h.WHAT, &[=:=], Hash[Str],
-        'typed optional hashes get their default' }();
+    # Inlined to avoid errors in forwarding arguments to other routines.
+    -> :@l {
+        cmp-ok @l.WHAT, &[=:=], Array, 'untyped optional arrays get their default Array...';
+        ok @l.DEFINITE, '...which is an instance';
+    }();
+    -> Str :@l {
+        cmp-ok @l.WHAT, &[=:=], Array[Str], 'typed optional arrays get their default parameterized Array...';
+        ok @l.DEFINITE, '...which is an instance';
+    }();
+    -> :%h {
+        cmp-ok %h.WHAT, &[=:=], Hash, 'untyped optional hashes get their default Hash...';
+        ok %h.DEFINITE, '...which is an instance';
+    }();
+    -> Str :%h {
+        cmp-ok %h.WHAT, &[=:=], Hash[Str], 'typed optional hashes get their default parameterized Hash...';
+        ok %h.DEFINITE, '...which is an instance';
+    }();
 }
 
 # vim: expandtab shiftwidth=4