role arguments can have constraints

Author: gfldex

doc/Language/typesystem.pod6

@@ -373,6 +373,15 @@ X<|Type Capture (role)>L<Type captures|/type/Signature#Type_Captures> are suppor
     dd $c;
     # OUTPUT«C $c = C.new(a => "default")␤»
 
+Arguments can have type constraints, C<where> clauses are not supported but can
+be achieved via C<subset>s.
+
+   class A {};
+   class B {};
+   subset A-or-B where * ~~ A|B;
+   role R[A-or-B ::T] {};
+   R[A.new].new;
+
 =head3 Versioning and Authorship
 
 Versioning and authorship can be applied via adverbs C«:ver<>» and C«:auth<>».