From 5ba487da5d4895139d1b2b49b436bf62fa93fb13 Mon Sep 17 00:00:00 2001
From: Sheehan Olver <solver@mac.com>
Date: Thu, 16 Mar 2023 10:53:06 +0000
Subject: [PATCH] Add *(::Diagonal, ::Diagonal, ::Diagonal) (#49005) (#49007)

(cherry picked from commit c37fc2798d9bb7349ff8eadb350ae68cf17cee61)
---
 stdlib/LinearAlgebra/src/diagonal.jl  |  6 ++++++
 stdlib/LinearAlgebra/test/diagonal.jl | 11 +++++++++++
 2 files changed, 17 insertions(+)

diff --git a/stdlib/LinearAlgebra/src/diagonal.jl b/stdlib/LinearAlgebra/src/diagonal.jl
index 291233ebe2e6a..6364169b2ba8a 100644
--- a/stdlib/LinearAlgebra/src/diagonal.jl
+++ b/stdlib/LinearAlgebra/src/diagonal.jl
@@ -355,6 +355,12 @@ function (*)(Da::Diagonal, A::AbstractMatrix, Db::Diagonal)
     return broadcast(*, Da.diag, A, permutedims(Db.diag))
 end
 
+function (*)(Da::Diagonal, Db::Diagonal, Dc::Diagonal)
+    _muldiag_size_check(Da, Db)
+    _muldiag_size_check(Db, Dc)
+    return Diagonal(Da.diag .* Db.diag .* Dc.diag)
+end
+
 # Get ambiguous method if try to unify AbstractVector/AbstractMatrix here using AbstractVecOrMat
 @inline mul!(out::AbstractVector, D::Diagonal, V::AbstractVector, alpha::Number, beta::Number) =
     _muldiag!(out, D, V, alpha, beta)
diff --git a/stdlib/LinearAlgebra/test/diagonal.jl b/stdlib/LinearAlgebra/test/diagonal.jl
index 83a2a896e736c..130a66ea0a1d5 100644
--- a/stdlib/LinearAlgebra/test/diagonal.jl
+++ b/stdlib/LinearAlgebra/test/diagonal.jl
@@ -1133,4 +1133,15 @@ Base.size(::SMatrix1) = (1, 1)
     @test C isa Matrix{SMatrix1{String}}
 end
 
+@testset "diagonal triple multiplication (#49005)" begin
+    n = 10
+    @test *(Diagonal(ones(n)), Diagonal(1:n), Diagonal(ones(n))) isa Diagonal
+    @test_throws DimensionMismatch (*(Diagonal(ones(n)), Diagonal(1:n), Diagonal(ones(n+1))))
+    @test_throws DimensionMismatch (*(Diagonal(ones(n)), Diagonal(1:n+1), Diagonal(ones(n+1))))
+    @test_throws DimensionMismatch (*(Diagonal(ones(n+1)), Diagonal(1:n), Diagonal(ones(n))))
+
+    # currently falls back to two-term *
+    @test *(Diagonal(ones(n)), Diagonal(1:n), Diagonal(ones(n)), Diagonal(1:n)) isa Diagonal
+end
+
 end # module TestDiagonal