allow creating an undef Diagonal (JuliaLang#38282)

Author: Roger-luo

stdlib/LinearAlgebra/src/diagonal.jl

@@ -63,6 +63,13 @@ AbstractMatrix{T}(D::Diagonal) where {T} = Diagonal{T}(D)
 Matrix(D::Diagonal) = diagm(0 => D.diag)
 Array(D::Diagonal) = Matrix(D)
 
+"""
+    Diagonal{T}(undef, n)
+
+Construct an uninitialized `Diagonal{T}` of length `n`. See `undef`.
+"""
+Diagonal{T}(::UndefInitializer, n::Integer) where T = Diagonal(Vector{T}(undef, n))
+
 # For D<:Diagonal, similar(D[, neweltype]) should yield a Diagonal matrix.
 # On the other hand, similar(D, [neweltype,] shape...) should yield a sparse matrix.
 # The first method below effects the former, and the second the latter.

stdlib/LinearAlgebra/test/diagonal.jl

@@ -744,6 +744,10 @@ end
     @test dot(zeros(Int32, 0), Diagonal(zeros(Int, 0)), zeros(Int16, 0)) === 0
 end
 
+@testset "Diagonal(undef)" begin
+    d = Diagonal{Float32}(undef, 2)
+    @test length(d.diag) == 2
+end
 
 @testset "permutedims (#39447)" begin
     for D in (Diagonal(zeros(5)), Diagonal(zeros(5) .+ 1im), Diagonal([[1,2],[3,4]]))