Define promote_rule for Diagonal matrices (#42142)

Co-authored-by: Daniel Karrasch <daniel.karrasch@posteo.de>

Author: jishnub

stdlib/LinearAlgebra/src/diagonal.jl

@@ -10,9 +10,17 @@ struct Diagonal{T,V<:AbstractVector{T}} <: AbstractMatrix{T}
         new{T,V}(diag)
     end
 end
+Diagonal{T,V}(d::Diagonal) where {T,V<:AbstractVector{T}} = Diagonal{T,V}(d.diag)
 Diagonal(v::AbstractVector{T}) where {T} = Diagonal{T,typeof(v)}(v)
 Diagonal{T}(v::AbstractVector) where {T} = Diagonal(convert(AbstractVector{T}, v)::AbstractVector{T})
 
+function Base.promote_rule(A::Type{<:Diagonal{<:Any,V}}, B::Type{<:Diagonal{<:Any,W}}) where {V,W}
+    X = promote_type(V, W)
+    T = eltype(X)
+    isconcretetype(T) && return Diagonal{T,X}
+    return typejoin(A, B)
+end
+
 """
     Diagonal(V::AbstractVector)
 
@@ -88,7 +96,7 @@ similar(D::Diagonal, ::Type{T}) where {T} = Diagonal(similar(D.diag, T))
 
 copyto!(D1::Diagonal, D2::Diagonal) = (copyto!(D1.diag, D2.diag); D1)
 
-size(D::Diagonal) = (length(D.diag),length(D.diag))
+size(D::Diagonal) = (n = length(D.diag); (n,n))
 
 function size(D::Diagonal,d::Integer)
     if d<1

stdlib/LinearAlgebra/test/diagonal.jl

@@ -884,6 +884,31 @@ end
     @test \(x, B) == /(B, x)
 end
 
+@testset "promotion" begin
+    for (v1, v2) in (([true], [1]), ([zeros(2,2)], [zeros(Int, 2,2)]))
+        T = promote_type(eltype(v1), eltype(v2))
+        V = promote_type(typeof(v1), typeof(v2))
+        d1 = Diagonal(v1)
+        d2 = Diagonal(v2)
+        v = [d1, d2]
+        @test (@inferred eltype(v)) == Diagonal{T, V}
+    end
+    # test for a type for which promote_type doesn't lead to a concrete eltype
+    struct MyArrayWrapper{T,N,A<:AbstractArray{T,N}} <: AbstractArray{T,N}
+       a :: A
+    end
+    Base.size(M::MyArrayWrapper) = size(M.a)
+    Base.axes(M::MyArrayWrapper) = axes(M.a)
+    Base.length(M::MyArrayWrapper) = length(M.a)
+    Base.getindex(M::MyArrayWrapper, i::Int...) = M.a[i...]
+    Base.setindex!(M::MyArrayWrapper, v, i::Int...) = M.a[i...] = v
+    d1 = Diagonal(MyArrayWrapper(1:3))
+    d2 = Diagonal(MyArrayWrapper(1.0:3.0))
+    c = [d1, d2]
+    @test c[1] == d1
+    @test c[2] == d2
+end
+
 @testset "zero and one" begin
     D1 = Diagonal(rand(3))
     @test D1 + zero(D1) == D1