Add similarmutable

Author: jishnub

stdlib/LinearAlgebra/src/generic.jl

@@ -388,6 +388,16 @@ function cross(a::AbstractVector, b::AbstractVector)
     [a2*b3-a3*b2, a3*b1-a1*b3, a1*b2-a2*b1]
 end
 
+"""
+    similarmutable(A)
+
+Return a mutable `AbstractArray` that shares its structure
+with `similar(A)`. The difference with `similar(A)` is that
+each index of the result corresponding to the stored ones
+of `A` will be mutable.
+"""
+similarmutable(A) = similar(A)
+
 """
     triu(M)
 
@@ -411,9 +421,9 @@ julia> triu(a)
 ```
 """
 function triu(M::AbstractMatrix)
-    d = similar(M)
-    A = triu!(d) # may return a different type
+    A = similarmutable(M)
     copytrito!(A, M, 'U')
+    triu!(A)
 end
 
 """
@@ -439,9 +449,9 @@ julia> tril(a)
 ```
 """
 function tril(M::AbstractMatrix)
-    d = similar(M)
-    A = tril!(d) # may return a different type
+    A = similarmutable(M)
     copytrito!(A, M, 'L')
+    tril!(A)
 end
 
 """

stdlib/LinearAlgebra/src/symmetric.jl

@@ -325,6 +325,8 @@ end
 # storage type of A (not wrapped in a symmetry type). The following method covers these cases.
 similar(A::Union{Symmetric,Hermitian}, ::Type{T}, dims::Dims{N}) where {T,N} = similar(parent(A), T, dims)
 
+similarmutable(A::Union{Symmetric, Hermitian}) = similar(parent(A))
+
 parent(A::HermOrSym) = A.data
 Symmetric{T,S}(A::Symmetric{T,S}) where {T,S<:AbstractMatrix{T}} = A
 Symmetric{T,S}(A::Symmetric) where {T,S<:AbstractMatrix{T}} = Symmetric{T,S}(convert(S,A.data),A.uplo)

stdlib/LinearAlgebra/src/tridiag.jl

@@ -157,6 +157,8 @@ axes(M::SymTridiagonal) = (ax = axes(M.dv, 1); (ax, ax))
 similar(S::SymTridiagonal, ::Type{T}) where {T} = SymTridiagonal(similar(S.dv, T), similar(S.ev, T))
 similar(S::SymTridiagonal, ::Type{T}, dims::Union{Dims{1},Dims{2}}) where {T} = similar(S.dv, T, dims)
 
+similarmutable(S::SymTridiagonal) = Tridiagonal(similar(_evview(S)), similar(S.dv), similar(_evview(S)))
+
 # copyto! for matching axes
 _copyto_banded!(dest::SymTridiagonal, src::SymTridiagonal) =
     (copyto!(dest.dv, src.dv); copyto!(dest.ev, _evview(src)); dest)