From 5a6b789a1745f7227dadea45a6eec0c0bf8f0d10 Mon Sep 17 00:00:00 2001
From: Jishnu Bhattacharya <jishnub.github@gmail.com>
Date: Wed, 22 May 2024 13:14:26 +0530
Subject: [PATCH] Copy parent to dest in general in copyto! for triangular
 (#54529)

---
 stdlib/LinearAlgebra/src/triangular.jl | 7 ++-----
 stdlib/LinearAlgebra/test/special.jl   | 9 +++++++++
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/stdlib/LinearAlgebra/src/triangular.jl b/stdlib/LinearAlgebra/src/triangular.jl
index ce2fb53498300..47595e06fd47d 100644
--- a/stdlib/LinearAlgebra/src/triangular.jl
+++ b/stdlib/LinearAlgebra/src/triangular.jl
@@ -533,10 +533,7 @@ function copyto!(dest::StridedMatrix, U::UpperOrLowerTriangular)
 end
 function _copyto!(dest::StridedMatrix, U::UpperOrLowerTriangular)
     copytrito!(dest, parent(U), U isa UpperOrUnitUpperTriangular ? 'U' : 'L')
-    _triangularize!(U)(dest)
-    if U isa Union{UnitUpperTriangular, UnitLowerTriangular}
-        dest[diagind(dest)] .= @view U[diagind(U, IndexCartesian())]
-    end
+    copytrito!(dest, U, U isa UpperOrUnitUpperTriangular ? 'L' : 'U')
     return dest
 end
 function _copyto!(dest::StridedMatrix, U::UpperOrLowerTriangular{<:Any, <:StridedMatrix})
@@ -545,7 +542,7 @@ function _copyto!(dest::StridedMatrix, U::UpperOrLowerTriangular{<:Any, <:Stride
     return dest
 end
 # for strided matrices, we explicitly loop over the arrays to improve cache locality
-# This fuses the copytrito! and triu/l operations
+# This fuses the copytrito! for the two halves
 function copyto_unaliased!(dest::StridedMatrix, U::UpperOrUnitUpperTriangular{<:Any, <:StridedMatrix})
     isunit = U isa UnitUpperTriangular
     for col in axes(dest,2)
diff --git a/stdlib/LinearAlgebra/test/special.jl b/stdlib/LinearAlgebra/test/special.jl
index be04fb564a6e8..a78767c68627e 100644
--- a/stdlib/LinearAlgebra/test/special.jl
+++ b/stdlib/LinearAlgebra/test/special.jl
@@ -128,6 +128,15 @@ Random.seed!(1)
         for M in (D, Bu, Bl, Tri, Sym)
             @test Matrix(M) == zeros(TypeWithZero, 3, 3)
         end
+
+        mutable struct MTypeWithZero end
+        Base.convert(::Type{MTypeWithZero}, ::TypeWithoutZero) = MTypeWithZero()
+        Base.convert(::Type{MTypeWithZero}, ::TypeWithZero) = MTypeWithZero()
+        Base.zero(x::MTypeWithZero) = zero(typeof(x))
+        Base.zero(::Type{MTypeWithZero}) = MTypeWithZero()
+        U = UpperTriangular(Symmetric(fill(TypeWithoutZero(), 2, 2)))
+        M = Matrix{MTypeWithZero}(U)
+        @test all(x -> x isa MTypeWithZero, M)
     end
 end