diff --git a/stdlib/LinearAlgebra/src/diagonal.jl b/stdlib/LinearAlgebra/src/diagonal.jl
index 4b7d9bd9d4af13..4be9f0ecef5b4b 100644
--- a/stdlib/LinearAlgebra/src/diagonal.jl
+++ b/stdlib/LinearAlgebra/src/diagonal.jl
@@ -77,8 +77,16 @@ Diagonal{T}(D::Diagonal{T}) where {T} = D
 Diagonal{T}(D::Diagonal) where {T} = Diagonal{T}(D.diag)
 
 AbstractMatrix{T}(D::Diagonal) where {T} = Diagonal{T}(D)
-Matrix(D::Diagonal) = diagm(0 => D.diag)
-Array(D::Diagonal) = Matrix(D)
+Matrix(D::Diagonal{T}) where {T} = Matrix{T}(D)
+Array(D::Diagonal{T}) where {T} = Matrix{T}(D)
+function Matrix{T}(D::Diagonal) where {T}
+    n = size(D, 1)
+    B = zeros(T, n, n)
+    for i in 1:n
+        B[i,i] = D.diag[i]
+    end
+    return B
+end
 
 """
     Diagonal{T}(undef, n)
diff --git a/stdlib/LinearAlgebra/src/special.jl b/stdlib/LinearAlgebra/src/special.jl
index ad2c97568c536d..beac0c524f2f4f 100644
--- a/stdlib/LinearAlgebra/src/special.jl
+++ b/stdlib/LinearAlgebra/src/special.jl
@@ -310,7 +310,7 @@ function _qlmul(Q::AbstractQ, B)
 end
 function _qlmul(adjQ::Adjoint{<:Any,<:AbstractQ}, B)
     TQB = promote_type(eltype(adjQ), eltype(B))
-    lmul!(adjoint(convert(AbstractMatrix{TQB}, parent(adjQ))), copy_similar(B, TQB))
+    lmul!(adjoint(convert(AbstractMatrix{TQB}, parent(adjQ))), Matrix{TQB}(B))
 end
 
 *(Q::AbstractQ, B::AbstractTriangular) = _qlmul(Q, B)
@@ -325,7 +325,7 @@ rmul!(A::AbstractTriangular, Q::Adjoint{<:Any,<:AbstractQ}) = rmul!(full!(A), Q)
 
 function _qrmul(A, Q::AbstractQ)
     TAQ = promote_type(eltype(A), eltype(Q))
-    return rmul!(copy_similar(A, TAQ), convert(AbstractMatrix{TAQ}, Q))
+    return rmul!(Matrix{TAQ}(A), convert(AbstractMatrix{TAQ}, Q))
 end
 function _qrmul(A, adjQ::Adjoint{<:Any,<:AbstractQ})
     Q = adjQ.parent
diff --git a/stdlib/LinearAlgebra/src/tridiag.jl b/stdlib/LinearAlgebra/src/tridiag.jl
index 5a3c7612f67844..f99e1951af01de 100644
--- a/stdlib/LinearAlgebra/src/tridiag.jl
+++ b/stdlib/LinearAlgebra/src/tridiag.jl
@@ -571,15 +571,14 @@ function size(M::Tridiagonal, d::Integer)
     end
 end
 
-function Matrix{T}(M::Tridiagonal{T}) where T
+function Matrix{T}(M::Tridiagonal) where {T}
     A = zeros(T, size(M))
-    for i = 1:length(M.d)
+    for i in 1:length(M.d)-1
         A[i,i] = M.d[i]
-    end
-    for i = 1:length(M.d)-1
         A[i+1,i] = M.dl[i]
         A[i,i+1] = M.du[i]
     end
+    A[n,n] = M.d[n]
     A
 end
 Matrix(M::Tridiagonal{T}) where {T} = Matrix{T}(M)