Allow sub-classing of Proxy objects

Inspired by:

  https://stackoverflow.com/questions/58055309/a-scalar-with-memory-or-how-to-correctly-tie

This allows one to say:

    class Foo is Proxy {
    }

and implement additional functionality to the Proxy object, like keeping
track which values a variable has had (from the SO question).

With much help from Jonathan Worthington.

src/Perl6/Metamodel/ClassHOW.nqp

@@ -21,6 +21,7 @@ class Perl6::Metamodel::ClassHOW
     does Perl6::Metamodel::BoolificationProtocol
     does Perl6::Metamodel::REPRComposeProtocol
     does Perl6::Metamodel::InvocationProtocol
+    does Perl6::Metamodel::ContainerSpecProtocol
     does Perl6::Metamodel::Finalization
     does Perl6::Metamodel::Concretization
 {
@@ -220,6 +221,7 @@ class Perl6::Metamodel::ClassHOW
         self.publish_type_cache($obj);
         self.publish_method_cache($obj);
         self.publish_boolification_spec($obj);
+        self.publish_container_spec($obj);
 
         # Compose the meta-methods.
         self.compose_meta_methods($obj);

src/Perl6/Metamodel/ContainerSpecProtocol.nqp

@@ -0,0 +1,34 @@
+role Perl6::Metamodel::ContainerSpecProtocol {
+    has $!code_pair;
+
+    method get_container_spec($obj) {
+        $!code_pair
+    }
+
+    method set_container_spec($obj, $code_pair) {
+        $!code_pair := $code_pair;
+    }
+
+    method publish_container_spec($obj) {
+        # If we have a container specification here, install that
+        if $!code_pair {
+            nqp::setcontspec($obj, 'code_pair', $!code_pair);
+            $obj.HOW.compose_repr($obj);
+        }
+
+        # look in the parents for any if none here
+        else {
+            for self.mro($obj) -> $class {
+                if nqp::can($class.HOW, 'get_container_spec') {
+                    my $code_pair := $class.HOW.get_container_spec($class);
+                    if $code_pair {
+                        $!code_pair := $code_pair;
+                        nqp::setcontspec($obj, 'code_pair', $!code_pair);
+                        $obj.HOW.compose_repr($obj);
+                        last;
+                    }
+                }
+            }
+        }
+    }
+}

src/Perl6/bootstrap.c/BOOTSTRAP.nqp

@@ -1749,17 +1749,16 @@ BEGIN {
         nqp::getattr($cont, Proxy, '&!STORE')($var, $val)
     })));
     Proxy.HOW.add_method(Proxy, 'new', nqp::getstaticcode(sub ($type, :$FETCH!, :$STORE!) {
-        my $cont := nqp::create(Proxy);
+        my $cont := nqp::create($type);
         nqp::bindattr($cont, Proxy, '&!FETCH', $FETCH);
         nqp::bindattr($cont, Proxy, '&!STORE', $STORE);
         $cont
     }));
-    Proxy.HOW.compose(Proxy);
-    nqp::setcontspec(Proxy, 'code_pair', nqp::hash(
+    Proxy.HOW.set_container_spec(Proxy, nqp::hash(
         'fetch', $PROXY_FETCH,
         'store', $PROXY_STORE
     ));
-    Proxy.HOW.compose_repr(Proxy);
+    Proxy.HOW.compose(Proxy);
 
     # Helper for creating a scalar attribute. Sets it up as a real Perl 6
     # Attribute instance, complete with container descriptor and optional

tools/templates/common_metamodel_sources

@@ -8,6 +8,7 @@ src/Perl6/Metamodel/LanguageRevision.nqp
 src/Perl6/Metamodel/TypePretense.nqp
 src/Perl6/Metamodel/MethodDelegation.nqp
 src/Perl6/Metamodel/BoolificationProtocol.nqp
+src/Perl6/Metamodel/ContainerSpecProtocol.nqp
 src/Perl6/Metamodel/PackageHOW.nqp
 src/Perl6/Metamodel/ModuleHOW.nqp
 src/Perl6/Metamodel/GenericHOW.nqp