Fix classical register visibility inside box scope

.pylintrc

@@ -104,10 +104,6 @@ recursive=no
 # source root.
 source-roots=
 
-# When enabled, pylint would attempt to guess common misconfiguration and emit
-# user-friendly hints instead of false-positive error messages.
-suggestion-mode=yes
-
 # Allow loading of arbitrary C extensions. Extensions are imported into the
 # active Python interpreter and may run arbitrary code.
 unsafe-load-any-extension=no

CHANGELOG.md

@@ -23,6 +23,7 @@ Types of changes:
 ### Removed
 
 ### Fixed
+- Fixed classical register declarations not being visible inside `box` scope, causing "Missing clbit register declaration" errors for measurement statements inside box blocks. ([#306](https://github.com/qBraid/pyqasm/pull/306))
 
 ### Dependencies
 

src/pyqasm/scope.py

@@ -193,12 +193,7 @@ def get_from_visible_scope(self, var_name: str) -> Variable | None:
         curr_scope = self.get_curr_scope()
         if self.in_global_scope():
             return global_scope.get(var_name, None)
-        if (
-            self.in_function_scope()
-            or self.in_gate_scope()
-            or self.in_box_scope()
-            or self.in_pulse_scope()
-        ):
+        if self.in_function_scope() or self.in_gate_scope() or self.in_pulse_scope():
             if var_name in curr_scope:
                 return curr_scope[var_name]
             if var_name in global_scope and (
@@ -207,7 +202,7 @@ def get_from_visible_scope(self, var_name: str) -> Variable | None:
                 # we also need to return the variable if it is a constant or qubit
                 # in the global scope, as it can be used in the function or gate
                 return global_scope[var_name]
-        if self.in_block_scope():
+        if self.in_block_scope() or self.in_box_scope():
             var_found = None
             for scope, context in zip(reversed(self._scope), reversed(self._context)):
                 if context != Context.BLOCK:

tests/qasm3/test_box.py

@@ -232,3 +232,53 @@ def test_box_statement_error(qasm_code, error_message, error_span, caplog):
             loads(qasm_code).unroll()
     assert error_message in str(err.value)
     assert error_span in caplog.text
+
+
+def test_box_measurement_with_classical_register():
+    """Measurement inside a box should have access to classical registers
+    declared in the enclosing (global) scope.
+
+    Regression: box scope was treated like function/gate scope, restricting
+    access to only constants and qubits from global scope. Classical registers
+    were invisible, causing 'Missing clbit register declaration' errors.
+    See: https://github.com/qBraid/pyqasm/issues/302
+    """
+    qasm_str = """
+    OPENQASM 3.0;
+    include "stdgates.inc";
+    qubit[4] q;
+    bit[3] c;
+
+    box [100ns] {
+        h q[0];
+        c[1] = measure q[1];
+    }
+    """
+    result = loads(qasm_str)
+    result.validate()
+
+
+def test_box_measurement_with_enclosing_block_scope():
+    """Measurement inside a box nested within a non-global scope (e.g. an
+    if-block) should still have access to classical registers declared in
+    the enclosing global scope.
+
+    Complements test_box_measurement_with_classical_register by ensuring the
+    scope walker traverses intermediate BLOCK contexts to reach the global
+    scope where the classical register is declared.
+    """
+    qasm_str = """
+    OPENQASM 3.0;
+    include "stdgates.inc";
+    qubit[4] q;
+    bit[3] c;
+
+    if (true) {
+        box {
+            h q[0];
+            c[1] = measure q[1];
+        }
+    }
+    """
+    result = loads(qasm_str)
+    result.validate()