diff --git a/stdlib/LinearAlgebra/src/triangular.jl b/stdlib/LinearAlgebra/src/triangular.jl
index 07eafaa8bfc3f..aba67f24a6b28 100644
--- a/stdlib/LinearAlgebra/src/triangular.jl
+++ b/stdlib/LinearAlgebra/src/triangular.jl
@@ -246,10 +246,11 @@ Base.isstored(A::UpperTriangular, i::Int, j::Int) =
 @propagate_inbounds getindex(A::UpperTriangular, i::Int, j::Int) =
     i <= j ? A.data[i,j] : _zero(A.data,j,i)
 
+_zero_triangular_half_str(::Type{<:UpperOrUnitUpperTriangular}) = "lower"
+_zero_triangular_half_str(::Type{<:LowerOrUnitLowerTriangular}) = "upper"
+
 @noinline function throw_nonzeroerror(T, @nospecialize(x), i, j)
-    _upper_lower_str(::Type{<:UpperOrUnitUpperTriangular}) = "upper"
-    _upper_lower_str(::Type{<:LowerOrUnitLowerTriangular}) = "lower"
-    Ts = _upper_lower_str(T)
+    Ts = _zero_triangular_half_str(T)
     Tn = nameof(T)
     throw(ArgumentError(
         lazy"cannot set index in the $Ts triangular part ($i, $j) of an $Tn matrix to a nonzero value ($x)"))
@@ -301,9 +302,7 @@ end
 end
 
 @noinline function throw_setindex_structuralzero_error(T, @nospecialize(x))
-    _struct_zero_half_str(::Type{<:UpperTriangular}) = "lower"
-    _struct_zero_half_str(::Type{<:LowerTriangular}) = "upper"
-    Ts = _struct_zero_half_str(T)
+    Ts = _zero_triangular_half_str(T)
     Tn = nameof(T)
     throw(ArgumentError(
         lazy"cannot set indices in the $Ts triangular part of an $Tn matrix to a nonzero value ($x)"))
@@ -510,24 +509,40 @@ tr(A::UnitLowerTriangular) = size(A, 1) * oneunit(eltype(A))
 tr(A::UpperTriangular) = tr(A.data)
 tr(A::UnitUpperTriangular) = size(A, 1) * oneunit(eltype(A))
 
+@propagate_inbounds function copyto!(dest::UpperOrLowerTriangular, U::UpperOrLowerTriangular)
+    if axes(dest) != axes(U)
+        @invoke copyto!(dest::AbstractArray, U::AbstractArray)
+    else
+        _copyto!(dest, U)
+    end
+    return dest
+end
+
 # copy and scale
-function copyto!(A::T, B::T) where {T<:Union{UpperTriangular,UnitUpperTriangular}}
+for T in (:UpperTriangular, :LowerTriangular)
+    @eval @inline function _copyto!(A::$T, B::$T)
+        @boundscheck checkbounds(A, axes(B)...)
+        copytrito!(parent(A), parent(B), uplo_char(A))
+        return A
+    end
+end
+@inline function _copyto!(A::UnitUpperTriangular, B::UnitUpperTriangular)
     @boundscheck checkbounds(A, axes(B)...)
     n = size(B,1)
     B2 = Base.unalias(A, B)
     for j = 1:n
-        for i = 1:(isa(B, UnitUpperTriangular) ? j-1 : j)
+        for i = 1:j-1
             @inbounds A[i,j] = B2[i,j]
         end
     end
     return A
 end
-function copyto!(A::T, B::T) where {T<:Union{LowerTriangular,UnitLowerTriangular}}
+@inline function _copyto!(A::UnitLowerTriangular, B::UnitLowerTriangular)
     @boundscheck checkbounds(A, axes(B)...)
     n = size(B,1)
     B2 = Base.unalias(A, B)
     for j = 1:n
-        for i = (isa(B, UnitLowerTriangular) ? j+1 : j):n
+        for i = j+1:n
             @inbounds A[i,j] = B2[i,j]
         end
     end
@@ -537,7 +552,7 @@ end
 _triangularize!(::UpperOrUnitUpperTriangular) = triu!
 _triangularize!(::LowerOrUnitLowerTriangular) = tril!
 
-function copyto!(dest::StridedMatrix, U::UpperOrLowerTriangular)
+@propagate_inbounds function copyto!(dest::StridedMatrix, U::UpperOrLowerTriangular)
     if axes(dest) != axes(U)
         @invoke copyto!(dest::StridedMatrix, U::AbstractArray)
     else
@@ -545,19 +560,20 @@ function copyto!(dest::StridedMatrix, U::UpperOrLowerTriangular)
     end
     return dest
 end
-function _copyto!(dest::StridedMatrix, U::UpperOrLowerTriangular)
+@propagate_inbounds function _copyto!(dest::StridedMatrix, U::UpperOrLowerTriangular)
     copytrito!(dest, parent(U), U isa UpperOrUnitUpperTriangular ? 'U' : 'L')
     copytrito!(dest, U, U isa UpperOrUnitUpperTriangular ? 'L' : 'U')
     return dest
 end
-function _copyto!(dest::StridedMatrix, U::UpperOrLowerTriangular{<:Any, <:StridedMatrix})
+@propagate_inbounds function _copyto!(dest::StridedMatrix, U::UpperOrLowerTriangular{<:Any, <:StridedMatrix})
     U2 = Base.unalias(dest, U)
     copyto_unaliased!(dest, U2)
     return dest
 end
 # for strided matrices, we explicitly loop over the arrays to improve cache locality
 # This fuses the copytrito! for the two halves
-function copyto_unaliased!(dest::StridedMatrix, U::UpperOrUnitUpperTriangular{<:Any, <:StridedMatrix})
+@inline function copyto_unaliased!(dest::StridedMatrix, U::UpperOrUnitUpperTriangular{<:Any, <:StridedMatrix})
+    @boundscheck checkbounds(dest, axes(U)...)
     isunit = U isa UnitUpperTriangular
     for col in axes(dest,2)
         for row in 1:col-isunit
@@ -569,7 +585,8 @@ function copyto_unaliased!(dest::StridedMatrix, U::UpperOrUnitUpperTriangular{<:
     end
     return dest
 end
-function copyto_unaliased!(dest::StridedMatrix, L::LowerOrUnitLowerTriangular{<:Any, <:StridedMatrix})
+@inline function copyto_unaliased!(dest::StridedMatrix, L::LowerOrUnitLowerTriangular{<:Any, <:StridedMatrix})
+    @boundscheck checkbounds(dest, axes(L)...)
     isunit = L isa UnitLowerTriangular
     for col in axes(dest,2)
         for row in 1:col-!isunit
diff --git a/stdlib/LinearAlgebra/test/triangular.jl b/stdlib/LinearAlgebra/test/triangular.jl
index 5a4f2fd0318d0..93077c5631390 100644
--- a/stdlib/LinearAlgebra/test/triangular.jl
+++ b/stdlib/LinearAlgebra/test/triangular.jl
@@ -1083,7 +1083,7 @@ end
 
 @testset "copyto! with aliasing (#39460)" begin
     M = Matrix(reshape(1:36, 6, 6))
-    @testset for T in (UpperTriangular, LowerTriangular)
+    @testset for T in (UpperTriangular, LowerTriangular, UnitUpperTriangular, UnitLowerTriangular)
         A = T(view(M, 1:5, 1:5))
         A2 = copy(A)
         B = T(view(M, 2:6, 2:6))
@@ -1091,6 +1091,25 @@ end
     end
 end
 
+@testset "copyto! with different sizes" begin
+    Ap = zeros(3,3)
+    Bp = rand(2,2)
+    @testset for T in (UpperTriangular, LowerTriangular, UnitUpperTriangular, UnitLowerTriangular)
+        A = T(Ap)
+        B = T(Bp)
+        @test_throws ArgumentError copyto!(A, B)
+    end
+    @testset "error message" begin
+        A = UpperTriangular(Ap)
+        B = UpperTriangular(Bp)
+        @test_throws "cannot set index in the lower triangular part" copyto!(A, B)
+
+        A = LowerTriangular(Ap)
+        B = LowerTriangular(Bp)
+        @test_throws "cannot set index in the upper triangular part" copyto!(A, B)
+    end
+end
+
 @testset "getindex with Integers" begin
     M = reshape(1:4,2,2)
     for Ttype in (UpperTriangular, UnitUpperTriangular)