noncliff: improve error handling in apply!(sm, pcT) (#374)

---------

Co-authored-by: Stefan Krastanov <[email protected]>

Author: Fe-r-oz

src/nonclifford.jl

@@ -183,6 +183,8 @@ end
 function _proj₊(sm::GeneralizedStabilizer, p::PauliOperator)
 end
 
+nqubits(sm::GeneralizedStabilizer) = nqubits(sm.stab)
+
 abstract type AbstractPauliChannel <: AbstractOperation end
 
 """A Pauli channel datastructure, mainly for use with [`GeneralizedStabilizer`](@ref)
@@ -232,6 +234,7 @@ nqubits(pc::PauliChannel) = nqubits(pc.paulis[1][1])
 See also: [`GeneralizedStabilizer`](@ref), [`PauliChannel`](@ref), [`UnitaryPauliChannel`](@ref)
 """
 function apply!(state::GeneralizedStabilizer, gate::AbstractPauliChannel; prune_threshold=1e-10)
+    nqubits(state) == nqubits(gate) || throw(DimensionMismatch(lazy"GeneralizedStabilizer has $(nqubits(state)) qubits, but PauliChannel has $(nqubits(gate)). Use `embed` to create an appropriately padded PauliChannel."))
     dict = state.destabweights
     stab = state.stab
     dtype = valtype(dict)

test/test_throws.jl

@@ -20,6 +20,7 @@ using InteractiveUtils: subtypes
 @test_throws DimensionMismatch mul_right!(P"X", P"XX")
 
 @test_throws ArgumentError GeneralizedStabilizer(S"XX")
+@test_throws DimensionMismatch apply!(GeneralizedStabilizer(random_stabilizer(2)), pcT)
 
 @test_throws ArgumentError UnitaryPauliChannel([P"X"], [1,2])
 @test_throws ArgumentError UnitaryPauliChannel([P"X",P"XX"], [1,2])