Reimplement compilation of subsets

lib/CodeGen.cs

@@ -4118,8 +4118,22 @@ public class DowncallReceiver : CallReceiver {
                 nst.mo.rtype   = type;
                 nst.mo.isPackage = (type == "package");
                 nst.mo.isRole    = (type == "role" || type == "prole");
+                nst.mo.isSubset  = (type == "subset");
 
                 return new Handle(nst);
+            } else if (cmd == "type_set_basetype") {
+                STable subset = (STable)Handle.Unbox(args[1]);
+                STable basety = (STable)Handle.Unbox(args[2]);
+
+                subset.mo.FillSubset(basety);
+                subset.initObject = basety.initObject;
+                subset.initVar = basety.initVar;
+                return null;
+            } else if (cmd == "type_set_where") {
+                STable  subset = (STable)Handle.Unbox(args[1]);
+                SubInfo where  = (SubInfo)Handle.Unbox(args[2]);
+                subset.mo.subsetWhereThunk = where.protosub;
+                return null;
             } else if (cmd == "create_sub") {
                 RuntimeUnit ru = (RuntimeUnit)Handle.Unbox(args[1]);
                 string name = (string)args[2];

src/niecza

@@ -21,6 +21,37 @@ use Sig;
 use STD;
 
 augment class NieczaActions {
+method type_declarator:subset ($/) {
+    my ($basetype) = self.process_name($*OFTYPE<longname>);
+    $basetype //= $*CURLEX<!sub>.compile_get_pkg('CORE', 'Any');
+    my @exports;
+
+    for map *.ast, @$<trait> -> $t {
+        if $t<export> {
+            push @exports, @( $t<export> );
+        } elsif $t<of> {
+            $basetype = $t<of>;
+        } else {
+            $/.CURSOR.sorry("Unsupported subset trait $t.keys()");
+        }
+    }
+
+    my $body = self.thunk_sub($<EXPR> ?? $<EXPR>.ast !! mklex($/, 'True'));
+
+    my ($lexvar, $obj) = "Any";
+
+    $/.CURSOR.trymop({
+        ($lexvar, $obj) = self.do_new_package($/, scope => $*SCOPE,
+            name => $<longname>, class => 'subset', :@exports);
+
+        $*CURLEX<!sub>.create_static_pad;
+
+        $obj.set_basetype($basetype);
+        $obj.set_where($body);
+    });
+
+    make mklex($/, $lexvar);
+}
 }
 
 CgOp._register_ops: < who sc_root sc_indir temporize _addmethod _invalidate