make one(::AbstractMatrix) use similar instead of zeros

base/abstractarray.jl

@@ -1221,6 +1221,23 @@ zero(x::AbstractArray{T}) where {T<:Number} = fill!(similar(x, typeof(zero(T))),
 zero(x::AbstractArray{S}) where {S<:Union{Missing, Number}} = fill!(similar(x, typeof(zero(S))), zero(S))
 zero(x::AbstractArray) = map(zero, x)
 
+function _one(unit::T, mat::AbstractMatrix) where {T}
+    (rows, cols) = axes(mat)
+    (length(rows) == length(cols)) ||
+      throw(DimensionMismatch("multiplicative identity defined only for square matrices"))
+    zer = zero(unit)::T
+    require_one_based_indexing(mat)
+    I = similar(mat, T)
+    fill!(I, zer)
+    for i ∈ rows
+        I[i, i] = unit
+    end
+    I
+end
+
+one(x::AbstractMatrix{T}) where {T} = _one(one(T), x)
+oneunit(x::AbstractMatrix{T}) where {T} = _one(oneunit(T), x)
+
 ## iteration support for arrays by iterating over `eachindex` in the array ##
 # Allows fast iteration by default for both IndexLinear and IndexCartesian arrays
 

base/array.jl

@@ -598,21 +598,6 @@ for (fname, felt) in ((:zeros, :zero), (:ones, :one))
     end
 end
 
-function _one(unit::T, x::AbstractMatrix) where T
-    require_one_based_indexing(x)
-    m,n = size(x)
-    m==n || throw(DimensionMismatch("multiplicative identity defined only for square matrices"))
-    # Matrix{T}(I, m, m)
-    I = zeros(T, m, m)
-    for i in 1:m
-        I[i,i] = unit
-    end
-    I
-end
-
-one(x::AbstractMatrix{T}) where {T} = _one(one(T), x)
-oneunit(x::AbstractMatrix{T}) where {T} = _one(oneunit(T), x)
-
 ## Conversions ##
 
 convert(::Type{T}, a::AbstractArray) where {T<:Array} = a isa T ? a : T(a)::T

test/abstractarray.jl

@@ -2119,3 +2119,20 @@ end
         end
     end
 end
+
+@testset "one" begin
+    @test one([1 2; 3 4]) == [1 0; 0 1]
+    @test one([1 2; 3 4]) isa Matrix{Int}
+
+    struct Mat <: AbstractMatrix{Int}
+        p::Matrix{Int}
+    end
+    Base.size(m::Mat) = size(m.p)
+    Base.IndexStyle(::Type{<:Mat}) = IndexLinear()
+    Base.getindex(m::Mat, i::Int) = m.p[i]
+    Base.setindex!(m::Mat, v, i::Int) = m.p[i] = v
+    Base.similar(::Mat, ::Type{Int}, size::NTuple{2,Int}) = Mat(Matrix{Int}(undef, size))
+
+    @test one(Mat([1 2; 3 4])) == Mat([1 0; 0 1])
+    @test one(Mat([1 2; 3 4])) isa Mat
+end