Add assert_decompose_ends_at_default_gateset consistency test (#5079)

* Add assert_decompose_ends_at_default_gateset consistency test

* Refactor assert_decompose_ends_at_default_gateset to provide hook to ignore gates without known decompositions

Author: tanujkhattar

cirq-core/cirq/protocols/decompose_protocol.py

@@ -47,6 +47,15 @@
 DecomposeResult = Union[None, NotImplementedType, 'cirq.OP_TREE']
 OpDecomposer = Callable[['cirq.Operation'], DecomposeResult]
 
+DECOMPOSE_TARGET_GATESET = ops.Gateset(
+    ops.XPowGate,
+    ops.YPowGate,
+    ops.ZPowGate,
+    ops.CZPowGate,
+    ops.MeasurementGate,
+    ops.GlobalPhaseGate,
+)
+
 
 def _value_error_describing_bad_operation(op: 'cirq.Operation') -> ValueError:
     return ValueError(f"Operation doesn't satisfy the given `keep` but can't be decomposed: {op!r}")

cirq-core/cirq/testing/__init__.py

@@ -33,6 +33,7 @@
 )
 
 from cirq.testing.consistent_decomposition import (
+    assert_decompose_ends_at_default_gateset,
     assert_decompose_is_consistent_with_unitary,
 )
 

cirq-core/cirq/testing/consistent_decomposition.py

@@ -47,3 +47,20 @@ def assert_decompose_is_consistent_with_unitary(val: Any, ignoring_global_phase:
     else:
         # coverage: ignore
         np.testing.assert_allclose(actual, expected, atol=1e-8)
+
+
+def _known_gate_with_no_decomposition(val: Any):
+    """Checks whether `val` is a known gate with no default decomposition to default gateset."""
+    return False
+
+
+def assert_decompose_ends_at_default_gateset(val: Any):
+    """Asserts that cirq.decompose(val) ends at default cirq gateset or a known gate."""
+    if _known_gate_with_no_decomposition(val):
+        return  # coverage: ignore
+    args = () if isinstance(val, ops.Operation) else (tuple(devices.LineQid.for_gate(val)),)
+    dec_once = protocols.decompose_once(val, [val(*args[0]) if args else val], *args)
+    for op in [*ops.flatten_to_ops(protocols.decompose(d) for d in dec_once)]:
+        assert _known_gate_with_no_decomposition(op.gate) or (
+            op in protocols.decompose_protocol.DECOMPOSE_TARGET_GATESET
+        ), f'{val} decomposed to {op}, which is not part of default cirq target gateset.'

cirq-core/cirq/testing/consistent_decomposition_test.py

@@ -15,6 +15,7 @@
 import pytest
 
 import numpy as np
+import sympy
 
 import cirq
 
@@ -49,3 +50,63 @@ def test_assert_decompose_is_consistent_with_unitary():
         cirq.testing.assert_decompose_is_consistent_with_unitary(
             BadGateDecompose().on(cirq.NamedQubit('q'))
         )
+
+
+class GateDecomposesToDefaultGateset(cirq.Gate):
+    def _num_qubits_(self):
+        return 2
+
+    def _decompose_(self, qubits):
+        return [GoodGateDecompose().on(qubits[0]), BadGateDecompose().on(qubits[1])]
+
+
+class GateDecomposeDoesNotEndInDefaultGateset(cirq.Gate):
+    def _num_qubits_(self):
+        return 4
+
+    def _decompose_(self, qubits):
+        yield GateDecomposeNotImplemented().on_each(*qubits)
+
+
+class GateDecomposeNotImplemented(cirq.SingleQubitGate):
+    def _decompose_(self, qubits):
+        return NotImplemented
+
+
+class ParameterizedGate(cirq.SingleQubitGate):
+    def _num_qubits_(self):
+        return 2
+
+    def _decompose_(self, qubits):
+        yield cirq.X(qubits[0]) ** sympy.Symbol("x")
+        yield cirq.Y(qubits[1]) ** sympy.Symbol("y")
+
+
+def test_assert_decompose_ends_at_default_gateset():
+
+    cirq.testing.assert_decompose_ends_at_default_gateset(GateDecomposesToDefaultGateset())
+    cirq.testing.assert_decompose_ends_at_default_gateset(
+        GateDecomposesToDefaultGateset().on(*cirq.LineQubit.range(2))
+    )
+
+    cirq.testing.assert_decompose_ends_at_default_gateset(ParameterizedGate())
+    cirq.testing.assert_decompose_ends_at_default_gateset(
+        ParameterizedGate().on(*cirq.LineQubit.range(2))
+    )
+
+    with pytest.raises(AssertionError):
+        cirq.testing.assert_decompose_ends_at_default_gateset(GateDecomposeNotImplemented())
+
+    with pytest.raises(AssertionError):
+        cirq.testing.assert_decompose_ends_at_default_gateset(
+            GateDecomposeNotImplemented().on(cirq.NamedQubit('q'))
+        )
+    with pytest.raises(AssertionError):
+        cirq.testing.assert_decompose_ends_at_default_gateset(
+            GateDecomposeDoesNotEndInDefaultGateset()
+        )
+
+    with pytest.raises(AssertionError):
+        cirq.testing.assert_decompose_ends_at_default_gateset(
+            GateDecomposeDoesNotEndInDefaultGateset().on(*cirq.LineQubit.range(4))
+        )