similar(::NemoMatrix, ::Ring) does not produce Nemo matrices

[lgoettgens]

julia> s = matrix(QQ, 2, 2, [1,2,3,4])
[1   2]
[3   4]

julia> t = similar(s, ZZ)
[#undef   #undef]
[#undef   #undef]

julia> typeof(s)
QQMatrix

julia> typeof(t)
AbstractAlgebra.Generic.MatSpaceElem{ZZRingElem}

I would expect t to be of type dense_matrix_type(ZZ) = ZZMatrix instead.

This is due to this line in AbstractAlgebra, but I am not sure how to easily fix this.
https://github.com/Nemocas/AbstractAlgebra.jl/blob/762eab1920547d2e4c3f3e77a4976fff119bf206/src/Matrix.jl#L348
(#1791 should make it easier to fix)

[fingolfin]

While dense_matrix_type(R)(R, r, c) does not currently work, this does:

parent_type(dense_matrix_type(R))(R, r, c)()

Thus we could replace

   M = Matrix{TT}(undef, (r, c))
   z = x isa MatElem ? Generic.MatSpaceElem{TT}(R, M) : Generic.MatRingElem{TT}(R, M)

by something like this, at least for now:

  if x isa MatElem
    return parent_type(dense_matrix_type(R))(R, r, c)()
  end
  M = Matrix{TT}(undef, (r, c))
  return Generic.MatRingElem{TT}(R, M)

[thofma]

Or zero_matrix(R, r, c), my all time favorite.