[Stretch] Support Duration type in classical expressions.

qiskit/circuit/classical/expr/visitors.py

@@ -299,8 +299,4 @@ def is_lvalue(node: expr.Expr, /) -> bool:
             >>> expr.is_lvalue(expr.bit_and(a, b))
             False
     """
-    if node.type.const:
-        # If the expression's resolution type is const, then this can never be
-        # an l-value (even if the expression is a Var).
-        return False
     return node.accept(_IS_LVALUE)

qiskit/circuit/quantumcircuit.py

@@ -2843,13 +2843,14 @@ def add_var(self, name_or_var: str | expr.Var, /, initial: typing.Any) -> expr.V
 
         Args:
             name_or_var: either a string of the variable name, or an existing instance of
-                :class:`~.expr.Var` to re-use.  Variables cannot shadow names that are already in
-                use within the circuit.
+                a non-const-typed :class:`~.expr.Var` to re-use.  Variables cannot shadow names
+                that are already in use within the circuit.
             initial: the value to initialize this variable with.  If the first argument was given
                 as a string name, the type of the resulting variable is inferred from the initial
                 expression; to control this more manually, either use :meth:`.Var.new` to manually
                 construct a new variable with the desired type, or use :func:`.expr.cast` to cast
-                the initializer to the desired type.
+                the initializer to the desired type. If a const-typed expression is provided, it
+                will be automatically cast to its non-const counterpart.
 
                 This must be either a :class:`~.expr.Expr` node, or a value that can be lifted to
                 one using :class:`.expr.lift`.
@@ -2859,7 +2860,8 @@ def add_var(self, name_or_var: str | expr.Var, /, initial: typing.Any) -> expr.V
             object will be returned.
 
         Raises:
-            CircuitError: if the variable cannot be created due to shadowing an existing variable.
+            CircuitError: if the variable cannot be created due to shadowing an existing variable
+                or a const variable was specified for ``name_or_var``.
 
         Examples:
             Define a new variable given just a name and an initializer expression::
@@ -2900,17 +2902,18 @@ def add_var(self, name_or_var: str | expr.Var, /, initial: typing.Any) -> expr.V
         # Validate the initializer first to catch cases where the variable to be declared is being
         # used in the initializer.
         circuit_scope = self._current_scope()
-        # Convenience method to widen Python integer literals to the right width during the initial
-        # lift, if the type is already known via the variable.
-        if (
-            isinstance(name_or_var, expr.Var)
-            and name_or_var.type.kind is types.Uint
-            and isinstance(initial, int)
-            and not isinstance(initial, bool)
-        ):
-            coerce_type = name_or_var.type
-        else:
-            coerce_type = None
+        coerce_type = None
+        if isinstance(name_or_var, expr.Var):
+            if name_or_var.type.const:
+                raise CircuitError("const variables are not supported.")
+            if (
+                name_or_var.type.kind is types.Uint
+                and isinstance(initial, int)
+                and not isinstance(initial, bool)
+            ):
+                # Convenience method to widen Python integer literals to the right width during
+                # the initial lift, if the type is already known via the variable.
+                coerce_type = name_or_var.type
         initial = _validate_expr(circuit_scope, expr.lift(initial, coerce_type))
         if isinstance(name_or_var, str):
             var = expr.Var.new(name_or_var, initial.type)
@@ -2961,6 +2964,8 @@ def add_uninitialized_var(self, var: expr.Var, /):
             raise CircuitError("cannot add an uninitialized variable in a control-flow scope")
         if not var.standalone:
             raise CircuitError("cannot add a variable wrapping a bit or register to a circuit")
+        if var.type.const:
+            raise CircuitError("const variables are not supported.")
         self._builder_api.add_uninitialized_var(var)
 
     def add_capture(self, var: expr.Var):
@@ -3023,8 +3028,14 @@ def add_input(  # pylint: disable=missing-raises-doc
             raise CircuitError("cannot add an input variable in a control-flow scope")
         if self._vars_capture:
             raise CircuitError("circuits to be enclosed with captures cannot have input variables")
-        if isinstance(name_or_var, expr.Var) and type_ is not None:
-            raise ValueError("cannot give an explicit type with an existing Var")
+        if isinstance(name_or_var, expr.Var):
+            if type_ is not None:
+                raise ValueError("cannot give an explicit type with an existing Var")
+            if name_or_var.type.const:
+                raise CircuitError("const variables are not supported")
+        elif type_ is not None and type_.const:
+            raise CircuitError("const variables are not supported")
+
         var = self._prepare_new_var(name_or_var, type_)
         self._vars_input[var.name] = var
         return var

qiskit/qpy/__init__.py

@@ -370,6 +370,30 @@ def open(*args):
 by ``num_circuits`` in the file header). There is no padding between the
 circuits in the data.
 
+.. _qpy_version_14:
+
+Version 14
+----------
+
+Version 14 adds support for additional :class:`~.types.Type` classes, and adds support for
+const-ness to existing types :class:`~.types.Bool` and :class:`~.types.Uint`.
+The ``EXPR_TYPE_BOOL`` and ``EXPR_TYPE_UNIT`` structs are now replaced by ``EXPR_TYPE_BOOL_V14``
+and ``EXPR_TYPE_UINT_V14``, respectively. See the updated expression type table below.
+
+EXPR_TYPE
+~~~~~~~~~
+
+A :class:`~.types.Type` is encoded by a single-byte ASCII ``char`` that encodes the kind of type,
+followed by a payload that varies depending on the type.  The defined codes are:
+
+======================  =========  =================================================================
+Qiskit class            Type code  Payload
+======================  =========  =================================================================
+:class:`~.types.Bool`   ``b``      One `_Bool const`.
+
+:class:`~.types.Uint`   ``u``      One ``uint32_t width``, followed by one ``_Bool const``.
+======================  =========  =================================================================
+
 .. _qpy_version_13:
 
 Version 13

qiskit/qpy/binary_io/circuits.py

@@ -1257,7 +1257,7 @@ def write_circuit(
         file_obj.write(metadata_raw)
         # Write header payload
         file_obj.write(registers_raw)
-        standalone_var_indices = value.write_standalone_vars(file_obj, circuit)
+        standalone_var_indices = value.write_standalone_vars(file_obj, circuit, version)
     else:
         if circuit.num_vars:
             raise exceptions.UnsupportedFeatureForVersion(
@@ -1425,7 +1425,7 @@ def read_circuit(file_obj, version, metadata_deserializer=None, use_symengine=Fa
             "q": [Qubit() for _ in out_bits["q"]],
             "c": [Clbit() for _ in out_bits["c"]],
         }
-    var_segments, standalone_var_indices = value.read_standalone_vars(file_obj, num_vars)
+    var_segments, standalone_var_indices = value.read_standalone_vars(file_obj, num_vars, version)
     circ = QuantumCircuit(
         out_bits["q"],
         out_bits["c"],