From 4fefccffa1f699d402438737cd9e9e1cb7b5809a Mon Sep 17 00:00:00 2001
From: Katharine Hyatt <kshyatt@users.noreply.github.com>
Date: Thu, 23 Sep 2021 06:12:19 -0400
Subject: [PATCH] Fix bug and add tests for Bidiagonal (#42354)

---
 stdlib/LinearAlgebra/src/bidiag.jl  |  2 +-
 stdlib/LinearAlgebra/test/bidiag.jl | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/stdlib/LinearAlgebra/src/bidiag.jl b/stdlib/LinearAlgebra/src/bidiag.jl
index 1a75c6a9287f06..e44def613dea34 100644
--- a/stdlib/LinearAlgebra/src/bidiag.jl
+++ b/stdlib/LinearAlgebra/src/bidiag.jl
@@ -735,7 +735,7 @@ function ldiv!(A::Bidiagonal, b::AbstractVector)
     end
 
     if N == 0
-        return x
+        return b
     end
 
     @inbounds begin
diff --git a/stdlib/LinearAlgebra/test/bidiag.jl b/stdlib/LinearAlgebra/test/bidiag.jl
index d9efdc1fd3ee60..274ca24fe018d5 100644
--- a/stdlib/LinearAlgebra/test/bidiag.jl
+++ b/stdlib/LinearAlgebra/test/bidiag.jl
@@ -269,6 +269,11 @@ Random.seed!(1)
                     end
                 end
             end
+            zdv = Vector{elty}(undef, 0)
+            zev = Vector{elty}(undef, 0)
+            zA  = Bidiagonal(zdv, zev, :U)
+            zb  = Vector{elty}(undef, 0)
+            @test ldiv!(zA, zb) === zb
         end
 
         if elty <: BlasReal
@@ -549,6 +554,14 @@ end
             B = Bidiagonal(dv, ev, uplo)
             @test dot(x, B, y) ≈ dot(B'x, y) ≈ dot(x, Matrix(B), y)
         end
+        dv = Vector{elty}(undef, 0)
+        ev = Vector{elty}(undef, 0)
+        x = Vector{elty}(undef, 0)
+        y = Vector{elty}(undef, 0)
+        for uplo in (:U, :L)
+            B = Bidiagonal(dv, ev, uplo)
+            @test dot(x, B, y) ≈ dot(zero(elty), zero(elty), zero(elty))
+        end
     end
 end