Sketch out SubsetHOW. Untested.

src/Perl6/Metamodel/EXPORTHOW.pm

@@ -11,4 +11,5 @@ my module EXPORTHOW {
     ($?PACKAGE.WHO)<role-group>   := Perl6::Metamodel::ParametricRoleGroupHOW;
     ($?PACKAGE.WHO)<role-curried> := Perl6::Metamodel::CurriedRoleHOW;
     ($?PACKAGE.WHO)<native>       := Perl6::Metamodel::NativeHOW;
+    ($?PACKAGE.WHO)<subset>       := Perl6::Metamodel::SubsetHOW;
 }

src/Perl6/Metamodel/SubsetHOW.pm

@@ -1,4 +1,41 @@
 class Perl6::Metamodel::SubsetHOW
     does Perl6::Metamodel::Naming
 {
+    # The subset type or nominal type that we refine.
+    has $!refinee;
+
+    # The block implementing the refinement.
+    has $!refinement;
+
+    method new_type(:$name = '<anon>', :$refinee!, :$refinement!) {
+        my $metasubset := self.new(:name($name), :refinee($refinee),
+            :refinement($refinement));
+        my $type := pir::repr_type_object_for__PPS($metasubset, 'Uninstantiable');
+        pir::stable_set_type_check_mode__0PI($type, 2)
+    }
+
+    method set_of($obj, $refinee) {
+        $!refinee := $refinee
+    }
+
+    method refinee($obj) {
+        $!refinee
+    }
+
+    method refinement($obj) {
+        $!refinement
+    }
+
+    # Do check when we're on LHS of smartmatch (e.g. Even ~~ Int).
+    method type_check($obj, $checkee) {
+        pir::perl6_booleanize__PI($checkee.HOW =:= self ||
+            pir::type_check__IPP($checkee, $!refinee))
+    }
+
+    # Here we check the value itself (when on RHS on smartmatch).
+    method accepts_type($obj, $checkee) {
+        pir::perl6_booleanize__PI(
+            pir::type_check__IPP($checkee, $!refinee) &&
+            $!refinement.ACCEPTS($checkee))
+    }
 }

src/core/EXPORTHOW.pm

@@ -12,4 +12,5 @@ my module EXPORTHOW {
     pir::set__vQsPP($?PACKAGE.WHO, Q:PIR{ %r = box 'role-group' }, Perl6::Metamodel::ParametricRoleGroupHOW);
     pir::set__vQsPP($?PACKAGE.WHO, Q:PIR{ %r = box 'role-curried' }, Perl6::Metamodel::CurriedRoleHOW);
     pir::set__vQsPP($?PACKAGE.WHO, Q:PIR{ %r = box 'native' }, Perl6::Metamodel::NativeHOW);
+    pir::set__vQsPP($?PACKAGE.WHO, Q:PIR{ %r = box 'subset' }, Perl6::Metamodel::SubsetHOW);
 }

tools/build/Makefile.in

@@ -102,6 +102,7 @@ METAMODEL_SOURCES = \
   src/Perl6/Metamodel/RoleToClassApplier.pm \
   src/Perl6/Metamodel/ClassHOW.pm \
   src/Perl6/Metamodel/NativeHOW.pm \
+  src/Perl6/Metamodel/SubsetHOW.pm \
   src/Perl6/Metamodel/ContainerDescriptor.pm \
   src/Perl6/Metamodel/BOOTSTRAP.pm \
   src/Perl6/Metamodel/EXPORTHOW.pm \