copy when flipping function type

hugr-py/src/hugr/_cfg.py

@@ -97,14 +97,16 @@ def add_block(self, input_types: TypeRow) -> Block:
         return new_block
 
     def add_successor(self, pred: Wire) -> Block:
-        pred = pred.out_port()
-        block = self.hugr._get_typed_op(pred.node, ops.DataflowBlock)
-        inputs = block.nth_outputs(pred.offset)
-        b = self.add_block(inputs)
+        b = self.add_block(self._nth_outputs(pred))
 
         self.branch(pred, b)
         return b
 
+    def _nth_outputs(self, wire: Wire) -> TypeRow:
+        port = wire.out_port()
+        block = self.hugr._get_typed_op(port.node, ops.DataflowBlock)
+        return block.nth_outputs(port.offset)
+
     def branch(self, src: Wire, dst: ToNode) -> None:
         # TODO check for existing link/type compatibility
         if dst.to_node() == self.exit:
@@ -116,8 +118,7 @@ def branch_exit(self, src: Wire) -> None:
         src = src.out_port()
         self.hugr.add_link(src, self.exit.inp(0))
 
-        src_block = self.hugr._get_typed_op(src.node, ops.DataflowBlock)
-        out_types = src_block.nth_outputs(src.offset)
+        out_types = self._nth_outputs(src)
         if self._exit_op._cfg_outputs is not None:
             if self._exit_op._cfg_outputs != out_types:
                 raise MismatchedExit(src.node.idx)

hugr-py/src/hugr/_tys.py

@@ -238,7 +238,7 @@ def empty(cls) -> FunctionType:
         return cls(input=[], output=[])
 
     def flip(self) -> FunctionType:
-        return FunctionType(input=self.output, output=self.input)
+        return FunctionType(input=list(self.output), output=list(self.input))
 
 
 @dataclass(frozen=True)