Repository: qBraid/pyqasm

Issue:
[BUG] Measurement in declaration `bit[4] = measure q;` fails
### Environment

- **PyQASM version**: 0.1.0
- **Python version**: 3.11.6
- **Operating system**: macOS 15.1.1


### What happened?

Unrolling the following program raises error - 

```python
In [1]: import pyqasm

In [2]: s = """
   ...: OPENQASM 3;
   ...: qubit q;
   ...: bit b = measure q;
   ...: """

In [3]: mod = pyqasm.loads(s)

In [4]: mod.unroll()
ERROR:root:Error at line 4, column 8 in QASM file
------------------------------------------
ValidationErrorTraceback (most recent call last)
Cell In[4], line 1
----> 1 mod.unroll()

File ~/Desktop/qBraid/envs/pyqasm/lib/python3.11/site-packages/pyqasm/modules/base.py:526, in QasmModule.unroll(self, **kwargs)
    524 self.num_qubits, self.num_clbits = -1, -1
    525 self._unrolled_ast = Program(statements=[], version=self.original_program.version)
--> 526 raise err

File ~/Desktop/qBraid/envs/pyqasm/lib/python3.11/site-packages/pyqasm/modules/base.py:521, in QasmModule.unroll(self, **kwargs)
    519     self.num_qubits, self.num_clbits = 0, 0
    520     visitor = QasmVisitor(module=self, **kwargs)
--> 521     self.accept(visitor)
    522 except (ValidationError, UnrollError) as err:
    523     # reset the unrolled ast and qasm
    524     self.num_qubits, self.num_clbits = -1, -1

File ~/Desktop/qBraid/envs/pyqasm/lib/python3.11/site-packages/pyqasm/modules/qasm3.py:47, in Qasm3Module.accept(self, visitor)
     41 def accept(self, visitor):
     42     """Accept a visitor for the module
     43 
     44     Args:
     45         visitor (QasmVisitor): The visitor to accept
     46     """
---> 47     unrolled_stmt_list = visitor.visit_basic_block(self._statements)
     48     final_stmt_list = visitor.finalize(unrolled_stmt_list)
     50     self._unrolled_ast.statements = final_stmt_list

File ~/Desktop/qBraid/envs/pyqasm/lib/python3.11/site-packages/pyqasm/visitor.py:1828, in QasmVisitor.visit_basic_block(self, stmt_list)
   1826 result = []
   1827 for stmt in stmt_list:
-> 1828     result.extend(self.visit_statement(stmt))
   1829 return result

File ~/Desktop/qBraid/envs/pyqasm/lib/python3.11/site-packages/pyqasm/visitor.py:1810, in QasmVisitor.visit_statement(self, statement)
   1808         result.extend(ret_stmts)
   1809     else:
-> 1810         result.extend(visitor_function(statement))  # type: ignore[operator]
   1811 else:
   1812     raise_qasm3_error(
   1813         f"Unsupported statement of type {type(statement)}", span=statement.span
   1814     )

File ~/Desktop/qBraid/envs/pyqasm/lib/python3.11/site-packages/pyqasm/visitor.py:1143, in QasmVisitor._visit_classical_declaration(self, statement)
   1139         init_value = self._evaluate_array_initialization(
   1140             statement.init_expression, final_dimensions, base_type
   1141         )
   1142     else:
-> 1143         init_value, stmts = Qasm3ExprEvaluator.evaluate_expression(
   1144             statement.init_expression
   1145         )
   1146         statements.extend(stmts)
   1148 variable = Variable(
   1149     var_name,
   1150     base_type,
   (...)
   1154     is_register=isinstance(base_type, qasm3_ast.BitType),
   1155 )

File ~/Desktop/qBraid/envs/pyqasm/lib/python3.11/site-packages/pyqasm/expressions.py:303, in Qasm3ExprEvaluator.evaluate_expression(cls, expression, const_expr, reqd_type, validate_only)
    300     statements.extend(ret_stmts)
    301     return _check_and_return_value(ret_value)
--> 303 raise_qasm3_error(
    304     f"Unsupported expression type {type(expression)}", ValidationError, expression.span
    305 )

File ~/Desktop/qBraid/envs/pyqasm/lib/python3.11/site-packages/pyqasm/exceptions.py:64, in raise_qasm3_error(message, err_type, span, raised_from)
     62 if raised_from:
     63     raise err_type(message) from raised_from
---> 64 raise err_type(message)

ValidationError: Unsupported expression type <class 'openqasm3.ast.QuantumMeasurement'>
```

### Suggestions (Optional)

The [`ClassicalDeclaration` handler](https://github.com/qBraid/pyqasm/blob/07ac98b6f894154e68039f58eef782745c29f592/src/pyqasm/visitor.py#L1056) should be updated to also account for measurements in initialization

Discussion:
@arulandu: Will work on this!

Solution patch:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3ab7268..cb71cd5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,23 @@ Types of changes:
 ## Unreleased
 
 ### Added
+- Added support for classical declarations with measurement ([#120](https://github.com/qBraid/pyqasm/pull/120)). Usage example -
+
+```python
+In [1]: from pyqasm import loads, dumps
+
+In [2]: module = loads(
+   ...: """OPENQASM 3.0;
+   ...: qubit q;
+   ...: bit b = measure q;
+   ...: """)
+
+In [3]: module.unroll()
+
+In [4]: dumps(module).splitlines()
+Out[4]: ['OPENQASM 3.0;', 'qubit[1] q;', 'bit[1] b;', 'b[0] = measure q[0];']
+```
+
 - Added support for `gphase`, `toffoli`, `not`, `c3sx` and `c4x` gates ([#86](https://github.com/qBraid/pyqasm/pull/86))
 - Added a `remove_includes` method to `QasmModule` to remove include statements from the generated QASM code ([#100](https://github.com/qBraid/pyqasm/pull/100)). Usage example - 
 
diff --git a/src/pyqasm/visitor.py b/src/pyqasm/visitor.py
index 78ff841..1a51de0 100644
--- a/src/pyqasm/visitor.py
+++ b/src/pyqasm/visitor.py
@@ -1134,6 +1134,11 @@ def _visit_classical_declaration(
                 init_value = self._evaluate_array_initialization(
                     statement.init_expression, final_dimensions, base_type
                 )
+            elif isinstance(statement.init_expression, qasm3_ast.QuantumMeasurement):
+                measurement, statement.init_expression = statement.init_expression, None
+                return self._visit_classical_declaration(statement) + self._visit_measurement(
+                    qasm3_ast.QuantumMeasurementStatement(measurement, statement.identifier)
+                )  # type: ignore
             else:
                 init_value, stmts = Qasm3ExprEvaluator.evaluate_expression(
                     statement.init_expression


Test patch:
diff --git a/tests/qasm3/test_measurement.py b/tests/qasm3/test_measurement.py
index 2872d5a..3c74502 100644
--- a/tests/qasm3/test_measurement.py
+++ b/tests/qasm3/test_measurement.py
@@ -128,6 +128,42 @@ def test_remove_measurement():
     check_unrolled_qasm(dumps(module), expected_qasm)
 
 
+def test_init_measure():
+    qasm3_string = """
+    OPENQASM 3.0;
+    qubit a;
+    qubit[2] b;
+    qubit[4] e;
+    bit c = measure a;
+    bit[2] d = measure b;
+    bit[2] f = measure e[:2];
+    bit[2] g = measure e[{2, 3}];
+    """
+
+    expected_qasm = """
+    OPENQASM 3.0;
+    qubit[1] a;
+    qubit[2] b;
+    qubit[4] e;
+    bit[1] c;
+    c[0] = measure a[0];
+    bit[2] d;
+    d[0] = measure b[0];
+    d[1] = measure b[1];
+    bit[2] f;
+    f[0] = measure e[0];
+    f[1] = measure e[1];
+    bit[2] g;
+    g[0] = measure e[2];
+    g[1] = measure e[3];
+    """
+
+    module = loads(qasm3_string)
+    module.unroll()
+    print(dumps(module))
+    check_unrolled_qasm(dumps(module), expected_qasm)
+
+
 def test_incorrect_measure():
     def run_test(qasm3_code, error_message):
         with pytest.raises(ValidationError, match=error_message):