From bfa01f9c482a7338c5dfcd4768080e52abd716ad Mon Sep 17 00:00:00 2001
From: Daniel Karrasch <daniel.karrasch@posteo.de>
Date: Tue, 23 Apr 2024 18:51:20 +0200
Subject: [PATCH] Fix generic triangular solves with empty matrices (#54201)

(cherry picked from commit 8945914d4f86cd38f1b079cfc916f881eddbb3c2)
---
 stdlib/LinearAlgebra/src/triangular.jl |  2 ++
 stdlib/LinearAlgebra/test/lu.jl        | 13 +++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/stdlib/LinearAlgebra/src/triangular.jl b/stdlib/LinearAlgebra/src/triangular.jl
index e8291b1081df2..3e87f6d346e2c 100644
--- a/stdlib/LinearAlgebra/src/triangular.jl
+++ b/stdlib/LinearAlgebra/src/triangular.jl
@@ -1131,6 +1131,7 @@ function generic_trimatdiv!(C::AbstractVecOrMat, uploc, isunitc, tfun::Function,
     if size(C) != size(B)
         throw(DimensionMismatch("size of output, $(size(C)), does not match size of right hand side, $(size(B))"))
     end
+    iszero(mA) && return C
     oA = oneunit(eltype(A))
     @inbounds if uploc == 'U'
         if isunitc == 'N'
@@ -1267,6 +1268,7 @@ function generic_trimatdiv!(C::AbstractVecOrMat, uploc, isunitc, ::Function, xA:
     if size(C) != size(B)
         throw(DimensionMismatch("size of output, $(size(C)), does not match size of right hand side, $(size(B))"))
     end
+    iszero(mA) && return C
     oA = oneunit(eltype(A))
     @inbounds if uploc == 'U'
         if isunitc == 'N'
diff --git a/stdlib/LinearAlgebra/test/lu.jl b/stdlib/LinearAlgebra/test/lu.jl
index 744cc543b993e..1797d8b5a9017 100644
--- a/stdlib/LinearAlgebra/test/lu.jl
+++ b/stdlib/LinearAlgebra/test/lu.jl
@@ -480,4 +480,17 @@ end
     LinearAlgebra.generic_lufact!(fill(Inf, 2, 2), check=false)
 end
 
+@testset "lu for empty matrices" begin
+    for T in (Float64, BigFloat)
+        A = fill(T(0.0), 0, 0)
+        v = fill(T(1.0), 0, 10)
+        @test A \ v ≈ lu(A) \ v
+        vt = permutedims(v)
+        @test vt / A ≈ vt / lu(A)
+        B = UpperTriangular(transpose(fill(complex(T(0.0)), 0, 0)'))
+        @test B \ v ≈ v
+        @test vt / B ≈ vt
+    end
+end
+
 end # module TestLU