Specialize one for the SizedArray test helper (#58209)

Since the size of the array is encoded in the type, we may define `one`
on the type. This is useful in certain linear algebra contexts.

(cherry picked from commit d9fafab)

Author: jishnub

test/abstractarray.jl

@@ -1907,3 +1907,36 @@ end
         @test r2[i] == z[j]
     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
+
+    @testset "SizedArray" begin
+        S = [1 2; 3 4]
+        A = SizedArrays.SizedArray{(2,2)}(S)
+        @test one(A) == one(typeof(A))
+        @test oneunit(A) == oneunit(typeof(A))
+        M = fill(A, 2, 2)
+        O = one(M)
+        for I in CartesianIndices(M)
+            if I[1] == I[2]
+                @test O[I] == one(S)
+            else
+                @test O[I] == zero(S)
+            end
+        end
+    end
+end

test/testhelpers/SizedArrays.jl

@@ -29,6 +29,12 @@ Base.size(a::SizedArray) = size(typeof(a))
 Base.size(::Type{<:SizedArray{SZ}}) where {SZ} = SZ
 Base.getindex(A::SizedArray, i...) = getindex(A.data, i...)
 Base.zero(::Type{T}) where T <: SizedArray = SizedArray{size(T)}(zeros(eltype(T), size(T)))
+function Base.one(::Type{SizedMatrix{SZ,T,A}}) where {SZ,T,A}
+    allequal(SZ) || throw(DimensionMismatch("multiplicative identity defined only for square matrices"))
+    D = diagm(fill(one(T), SZ[1]))
+    SizedArray{SZ}(convert(A, D))
+end
+Base.parent(S::SizedArray) = S.data
 +(S1::SizedArray{SZ}, S2::SizedArray{SZ}) where {SZ} = SizedArray{SZ}(S1.data + S2.data)
 ==(S1::SizedArray{SZ}, S2::SizedArray{SZ}) where {SZ} = S1.data == S2.data
 function *(S1::SizedArray, S2::SizedArray)