FullPeepholeOptimise can increase the 2-qubit gate count for symbolic circuits

[cqc-alec]

from sympy import Symbol
from pytket.circuit import Circuit, OpType
from pytket.passes import FullPeepholeOptimise

a, b, c = Symbol("a"), Symbol("b"), Symbol("c")
c = Circuit(2).TK2(a, b, c, 0, 1)
FullPeepholeOptimise(target_2qb_gate=OpType.TK2).apply(c)
print(c.n_gates_of_type(OpType.TK2))

Output:

3

The reason is that inside FullPeepholeOptimise is a clifford_simp transform, and inside that there is a step to decompose all multi-qubit gates to CX. The later squashing step cannot squash these back again because they contain symbolic gates in between.

[github-actions]

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