Raise a validation error for stretch arithmetic instead of a TypeError

CHANGELOG.md

@@ -29,6 +29,7 @@ Types of changes:
 ### Removed
 
 ### Fixed
+- Fixed a `duration` initialized from arithmetic on a `stretch` operand, such as `duration b = a + s;` or `2 * s`, raising a bare `TypeError: unsupported operand type(s) for +: 'float' and 'NoneType'`. A `stretch` has no value until scheduling, so it now raises a `ValidationError` naming the variable ([#417](https://github.com/qBraid/pyqasm/pull/417), [#389](https://github.com/qBraid/pyqasm/issues/389)).
 - Decompose `p`, `phaseshift`, and `u1` (`U1`) gates into a single `rz(theta)` rotation instead of `h; rx(theta); h` ([#470](https://github.com/qBraid/pyqasm/pull/470), [#469](https://github.com/qBraid/pyqasm/issues/469)).
 - Fixed `rebase` failing on `ccx`, `id`, and the `p` gates produced by unrolling `c3x` for the `ROTATIONAL_CX` / `CLIFFORD_T` basis sets ([#468](https://github.com/qBraid/pyqasm/pull/468), [#467](https://github.com/qBraid/pyqasm/issues/467)).
 - `loads()` syntax errors now name the offending token, line and column, e.g. `Failed to parse OpenQASM string: syntax error near ';' at line 3, column 13`. Before, the reason after the colon was empty ([#460](https://github.com/qBraid/pyqasm/pull/460)).

src/pyqasm/expressions.py

@@ -471,6 +471,19 @@ def _get_external_function_return_type(expression):
                 return (None, [])
 
             statements.extend(rhs_statements)
+            for operand in (expression.lhs, expression.rhs):
+                if not isinstance(operand, Identifier):
+                    continue
+                assert cls.visitor_obj
+                var = cls.visitor_obj._scope_manager.get_from_visible_scope(operand.name)
+                # const stretch declarations carry a value, so only reject unresolved ones
+                if Qasm3Validator.validate_variable_type(var, StretchType) and var.value is None:
+                    raise_qasm3_error(
+                        f"Arithmetic on 'stretch' operand '{operand.name}' is not yet supported",
+                        err_type=ValidationError,
+                        error_node=expression,
+                        span=expression.span,
+                    )
             return _check_and_return_value(
                 qasm3_expression_op_map(expression.op.name, lhs_value, rhs_value)
             )

tests/qasm3/declarations/test_classical.py

@@ -518,6 +518,64 @@ def test_stretch_type_error(qasm_code, error_message, error_span, caplog):
     assert error_span in caplog.text
 
 
+@pytest.mark.parametrize(
+    "qasm_code,stretch_var,error_span",
+    [
+        (
+            """
+            OPENQASM 3.0;
+            include "stdgates.inc";
+            stretch s;
+            duration a = 300ns;
+            duration b = a + s;
+            """,
+            "s",
+            r"Error at line 6, column 25",
+        ),
+        (
+            """
+            OPENQASM 3.0;
+            include "stdgates.inc";
+            stretch s;
+            duration b = 2 * s;
+            """,
+            "s",
+            r"Error at line 5, column 25",
+        ),
+        (
+            """
+            OPENQASM 3.0;
+            include "stdgates.inc";
+            stretch s;
+            duration a = 300ns;
+            duration b = a + 2 * s;
+            """,
+            "s",
+            r"Error at line 6, column 29",
+        ),
+    ],
+)
+def test_stretch_arithmetic_not_supported(qasm_code, stretch_var, error_span, caplog):
+    with pytest.raises(ValidationError) as err:
+        with caplog.at_level("ERROR"):
+            loads(qasm_code).validate()
+    cause = err.value.__cause__
+    assert cause is not None
+    assert f"Arithmetic on 'stretch' operand '{stretch_var}' is not yet supported" in str(cause)
+    assert error_span in caplog.text
+
+
+def test_const_stretch_arithmetic():
+    qasm3_string = """
+    OPENQASM 3.0;
+    include "stdgates.inc";
+    const stretch s = 100ns;
+    duration a = 300ns;
+    duration b = a + 2 * s;
+    """
+    loads(qasm3_string).validate()
+
+
 @pytest.mark.parametrize(
     "qasm_code,error_message,error_span",
     [