Special case empty covec-diagonal-vec product (#35557)

Co-Authored-By: Takafumi Arakaki <takafumi.a@gmail.com>

stdlib/LinearAlgebra/src/diagonal.jl

@@ -659,14 +659,19 @@ end
 # disambiguation methods: * of Diagonal and Adj/Trans AbsVec
 *(x::Adjoint{<:Any,<:AbstractVector}, D::Diagonal) = Adjoint(map((t,s) -> t'*s, D.diag, parent(x)))
 *(x::Transpose{<:Any,<:AbstractVector}, D::Diagonal) = Transpose(map((t,s) -> transpose(t)*s, D.diag, parent(x)))
-*(x::Adjoint{<:Any,<:AbstractVector}, D::Diagonal, y::AbstractVector) =
-    mapreduce(t -> t[1]*t[2]*t[3], +, zip(x, D.diag, y))
-*(x::Transpose{<:Any,<:AbstractVector}, D::Diagonal, y::AbstractVector) =
-    mapreduce(t -> t[1]*t[2]*t[3], +, zip(x, D.diag, y))
-function dot(x::AbstractVector, D::Diagonal, y::AbstractVector)
-    mapreduce(t -> dot(t[1], t[2], t[3]), +, zip(x, D.diag, y))
+*(x::Adjoint{<:Any,<:AbstractVector},   D::Diagonal, y::AbstractVector) = _mapreduce_prod(*, x, D, y)
+*(x::Transpose{<:Any,<:AbstractVector}, D::Diagonal, y::AbstractVector) = _mapreduce_prod(*, x, D, y)
+dot(x::AbstractVector, D::Diagonal, y::AbstractVector) = _mapreduce_prod(dot, x, D, y)
+
+function _mapreduce_prod(f, x, D::Diagonal, y)
+    if isempty(x) && isempty(D) && isempty(y)
+        return zero(Base.promote_op(f, eltype(x), eltype(D), eltype(y)))
+    else
+        return mapreduce(t -> f(t[1], t[2], t[3]), +, zip(x, D.diag, y))
+    end
 end
 
+
 function cholesky!(A::Diagonal, ::Val{false} = Val(false); check::Bool = true)
     info = 0
     for (i, di) in enumerate(A.diag)

stdlib/LinearAlgebra/test/diagonal.jl

@@ -711,4 +711,10 @@ end
     @test s1 == prod(sign, d)
 end
 
+@testset "Empty (#35424)" begin
+    @test zeros(0)'*Diagonal(zeros(0))*zeros(0) === 0.0
+    @test transpose(zeros(0))*Diagonal(zeros(Complex{Int}, 0))*zeros(0) === 0.0 + 0.0im
+    @test dot(zeros(Int32, 0), Diagonal(zeros(Int, 0)), zeros(Int16, 0)) === 0
+end
+
 end # module TestDiagonal