Improve compose() errors for circuit width mismatches

qiskit/circuit/quantumcircuit.py

@@ -2280,9 +2280,15 @@ def replace_var(
                 dest.append(other, qargs=qubits, cargs=clbits, copy=copy)
             return None if inplace else dest
 
-        if other.num_qubits > dest.num_qubits or other.num_clbits > dest.num_clbits:
+        if other.num_qubits > dest.num_qubits:
             raise CircuitError(
-                "Trying to compose with another QuantumCircuit which has more 'in' edges."
+                f"Cannot compose a circuit with {other.num_qubits} qubit(s) "
+                f"into a circuit with {dest.num_qubits} qubit(s)."
+            )
+        if other.num_clbits > dest.num_clbits:
+            raise CircuitError(
+                f"Cannot compose a circuit with {other.num_clbits} classical bit(s) "
+                f"into a circuit with {dest.num_clbits} classical bit(s)."
             )
 
         # Maps bits in 'other' to bits in 'dest'.

test/python/circuit/test_compose.py

@@ -972,6 +972,31 @@ def test_rejects_duplicate_bits(self):
         with self.assertRaisesRegex(CircuitError, "Duplicate clbits"):
             base.compose(attempt, [0, 1], [1, 1])
 
+    def test_rejects_rhs_with_too_many_qubits(self):
+        """Test that compose rejects a circuit with too many qubits."""
+        base = QuantumCircuit(1)
+        attempt = QuantumCircuit(2)
+
+        with self.assertRaisesRegex(
+            CircuitError,
+            r"Cannot compose a circuit with 2 qubit\(s\) into a circuit with 1 qubit\(s\)\.",
+        ):
+            base.compose(attempt)
+
+    def test_rejects_rhs_with_too_many_clbits(self):
+        """Test that compose rejects a circuit with too many classical bits."""
+        base = QuantumCircuit(1, 1)
+        attempt = QuantumCircuit(1, 2)
+
+        with self.assertRaisesRegex(
+            CircuitError,
+            (
+                r"Cannot compose a circuit with 2 classical bit\(s\) "
+                r"into a circuit with 1 classical bit\(s\)\."
+            ),
+        ):
+            base.compose(attempt)
+
     def test_cannot_mix_inputs_and_captures(self):
         """The rules about mixing `input` and `capture` vars should still apply."""
         a = expr.Var.new("a", types.Bool())