Fix stim.Circuit.to_tableau ignoring SPP and SPP_DAG

- Fix `SPP` not being classified as a unitary gate
- Fix `SPP_DAG` not being classified as a unitary gate

Fixes #846

src/stim/circuit/circuit_pybind_test.py

@@ -864,7 +864,6 @@ def test_likeliest_error_sat_problem():
         DETECTOR rec[-1] rec[-2]
     """)
     sat_str = c.likeliest_error_sat_problem(quantization=100)
-    print(sat_str)
     assert sat_str == 'p wcnf 2 4 401\n18 -1 0\n100 -2 0\n401 -1 0\n401 2 0\n'
 
 
@@ -1902,3 +1901,14 @@ def test_pop():
 def test_circuit_create_with_odd_cx():
     with pytest.raises(ValueError, match="0, 1, 2"):
         stim.Circuit("CX 0 1 2")
+
+
+def test_to_tableau():
+    assert stim.Circuit().to_tableau() == stim.Tableau(0)
+    assert stim.Circuit("QUBIT_COORDS 0").to_tableau() == stim.Tableau(1)
+    assert stim.Circuit("I 0").to_tableau() == stim.Tableau(1)
+    assert stim.Circuit("H 0").to_tableau() == stim.Tableau.from_named_gate("H")
+    assert stim.Circuit("CX 0 1").to_tableau() == stim.Tableau.from_named_gate("CX")
+    assert stim.Circuit("SPP Z0").to_tableau() == stim.Tableau.from_named_gate("S")
+    assert stim.Circuit("SPP X0").to_tableau() == stim.Tableau.from_named_gate("SQRT_X")
+    assert stim.Circuit("SPP_DAG Y0*Y1").to_tableau() == stim.Tableau.from_named_gate("SQRT_YY_DAG")

src/stim/gates/gate_data_pauli_product.cc

@@ -99,7 +99,7 @@ S 1
             .id = GateType::SPP,
             .best_candidate_inverse_id = GateType::SPP_DAG,
             .arg_count = 0,
-            .flags = (GateFlags)(GATE_TARGETS_PAULI_STRING | GATE_TARGETS_COMBINERS),
+            .flags = (GateFlags)(GATE_TARGETS_PAULI_STRING | GATE_TARGETS_COMBINERS | GATE_IS_UNITARY),
             .category = "P_Generalized Pauli Product Gates",
             .help = R"MARKDOWN(
 The generalized S gate. Phases the -1 eigenspace of Pauli product observables by i.
@@ -174,7 +174,7 @@ CX 2 1
             .id = GateType::SPP_DAG,
             .best_candidate_inverse_id = GateType::SPP,
             .arg_count = 0,
-            .flags = (GateFlags)(GATE_TARGETS_PAULI_STRING | GATE_TARGETS_COMBINERS),
+            .flags = (GateFlags)(GATE_TARGETS_PAULI_STRING | GATE_TARGETS_COMBINERS | GATE_IS_UNITARY),
             .category = "P_Generalized Pauli Product Gates",
             .help = R"MARKDOWN(
 The generalized S_DAG gate. Phases the -1 eigenspace of Pauli product observables by -i.