Change similar() to use UndefInitializer constructors

This is incompatible with older Nemo versions.

src/Matrix.jl

@@ -395,7 +395,7 @@ end
 Create an uninitialized matrix over the given ring and dimensions,
 with defaults based upon the given source matrix `x`.
 """
-similar(x::MatElem, R::NCRing, r::Int, c::Int) = zero_matrix(R, r, c)
+similar(x::MatElem, R::NCRing, r::Int, c::Int) = dense_matrix_type(R)(R, undef, r, c)
 
 similar(x::MatElem, R::NCRing) = similar(x, R, nrows(x), ncols(x))
 

src/generic/Matrix.jl

@@ -25,6 +25,10 @@ parent(a::MatElem) = matrix_space(base_ring(a), nrows(a), ncols(a))
 
 Return the type of matrices with coefficients of type `T` respectively
 `elem_type(S)`.
+
+Implementations of the ring interface only need to provide a method
+for the argument a subtype of `NCRingElement`; the other variants are
+implemented by calling that method.
 """
 dense_matrix_type(::T) where T <: NCRing = dense_matrix_type(elem_type(T))
 dense_matrix_type(::T) where T <: NCRingElement = dense_matrix_type(T)

test/generic/Matrix-test.jl

@@ -73,11 +73,13 @@ struct F2Matrix <: AbstractAlgebra.MatElem{F2Elem}
    m::Generic.MatSpaceElem{F2Elem}
 end
 
+F2Matrix(::F2, ::UndefInitializer, r::Int, c::Int) = F2Matrix(Generic.MatSpaceElem{F2Elem}(F2(), undef, r, c))
+
 AbstractAlgebra.elem_type(::Type{F2MatSpace}) = F2Matrix
 AbstractAlgebra.parent_type(::Type{F2Matrix}) = F2MatSpace
 
 AbstractAlgebra.base_ring(::F2MatSpace) = F2()
-AbstractAlgebra.dense_matrix_type(::Type{F2}) = F2Matrix
+AbstractAlgebra.dense_matrix_type(::Type{F2Elem}) = F2Matrix
 AbstractAlgebra.matrix_space(::F2, r::Int, c::Int) = F2MatSpace(F2(), r, c)
 
 AbstractAlgebra.number_of_rows(a::F2Matrix) = nrows(a.m)