fix #191: too restrictive typeassert for MixedDestabilizer (#366)

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

src/QuantumClifford.jl

@@ -571,8 +571,8 @@ function MixedDestabilizer(stab::Stabilizer{T}; undoperm=true, reportperm=false)
         t[n+r+s+1:end] = sZ # The other logical set in the tableau
     end
     if undoperm
-        t = t[:,invperm(permx[permz])]::T
-        return MixedDestabilizer(t, r+s)::MixedDestabilizer{T}
+        t = t[:,invperm(permx[permz])]
+        return MixedDestabilizer(t, r+s)
     end
     if reportperm
         return (MixedDestabilizer(t, r+s)::MixedDestabilizer{T}, r, permx, permz)

test/test_stabs.jl

@@ -1,4 +1,5 @@
 @testitem "Stabilizers" begin
+    using QuantumClifford
     using QuantumClifford: stab_looks_good, destab_looks_good, mixed_stab_looks_good, mixed_destab_looks_good
     test_sizes = [1,2,10,63,64,65,127,128,129] # Including sizes that would test off-by-one errors in the bit encoding.
     @testset "Pure and Mixed state initialization" begin
@@ -107,4 +108,17 @@
         @test hcat(copy(tab(s1)), copy(tab(s2))) == T"-YZZY XXYX"
         @test hcat(copy(tab(s1)), copy(tab(s2)), copy(tab(s1)), copy(tab(s2))) == T"YZZYYZZY XXYXXXYX"
     end
+
+    @testset "MixedDestabilizer over subarrays (#191)" begin
+        # Case 1: QuantumClifford.Tableau{Vector{UInt8}, Matrix{UInt64}}
+        n = 6
+        stab = random_stabilizer(n)
+        regular_arr = MixedDestabilizer(stab; undoperm=true)
+        @test isa(regular_arr, MixedDestabilizer)
+        # Case 2: Tableau{SubArray{...}, SubArray{...}, Tuple{Base.Slice{...}}}
+        stab = random_stabilizer(n)
+        substab = @view stab[3:n]
+        md_via_subarr = MixedDestabilizer(substab; undoperm=true)
+        @test isa(md_via_subarr, MixedDestabilizer)
+    end
 end