add a Diagonal constructor and a conversion (#48895)

* add a Diagonal constructor and a conversion Co-authored-by: woclass <[email protected]>
JuliaLang · Mar 9, 2023 · 2ed1cfe · 2ed1cfe
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diff --git a/stdlib/LinearAlgebra/src/diagonal.jl b/stdlib/LinearAlgebra/src/diagonal.jl
@@ -96,6 +96,11 @@ julia> diag(A, 2)
 ```
 """
 Diagonal(A::AbstractMatrix) = Diagonal(diag(A))
+Diagonal{T}(A::AbstractMatrix) where T = Diagonal{T}(diag(A))
+function convert(::Type{T}, A::AbstractMatrix) where T<:Diagonal
+    checksquare(A)
+    isdiag(A) ? T(A) : throw(InexactError(:convert, T, A))
+end
 
 Diagonal(D::Diagonal) = D
 Diagonal{T}(D::Diagonal{T}) where {T} = D

diff --git a/stdlib/LinearAlgebra/test/diagonal.jl b/stdlib/LinearAlgebra/test/diagonal.jl
@@ -43,11 +43,14 @@ Random.seed!(1)
         end
         @test eltype(Diagonal{elty}([1,2,3,4])) == elty
         @test isa(Diagonal{elty,Vector{elty}}(GenericArray([1,2,3,4])), Diagonal{elty,Vector{elty}})
+        @test isa(Diagonal{elty}(rand(Int,n,n)), Diagonal{elty,Vector{elty}})
         DI = Diagonal([1,2,3,4])
         @test Diagonal(DI) === DI
         @test isa(Diagonal{elty}(DI), Diagonal{elty})
         # issue #26178
-        @test_throws MethodError convert(Diagonal, [1, 2, 3, 4])
+        @test_throws MethodError convert(Diagonal, [1,2,3,4])
+        @test_throws DimensionMismatch convert(Diagonal, [1 2 3 4])
+        @test_throws InexactError convert(Diagonal, ones(2,2))
     end
 
     @testset "Basic properties" begin