Generic fallback for fillstored! (#1389)

Also, specialize for `Adjoint`/`Transpose` to dispatch to the methods
for the parent.

The following works after this:
```julia
julia> U = Adjoint(UpperTriangular(zeros(2,2)))
2×2 adjoint(::UpperTriangular{Float64, Matrix{Float64}}) with eltype Float64:
 0.0   ⋅ 
 0.0  0.0

julia> LinearAlgebra.fillstored!(U, 3)
2×2 adjoint(::UpperTriangular{Float64, Matrix{Float64}}) with eltype Float64:
 3.0   ⋅ 
 3.0  3.0
```

---------

Co-authored-by: Viral B. Shah <viral@juliacomputing.com>

Author: jishnub

src/adjtrans.jl

@@ -582,6 +582,11 @@ diagview(A::Adjoint, k::Integer = 0) = _vecadjoint(diagview(parent(A), -k))
 triu!(A::AdjOrTransAbsMat, k::Integer = 0) = wrapperop(A)(tril!(parent(A), -k))
 tril!(A::AdjOrTransAbsMat, k::Integer = 0) = wrapperop(A)(triu!(parent(A), -k))
 
+function fillstored!(A::AdjOrTransAbsMat, v)
+    fillstored!(parent(A), wrapperop(A)(v))
+    return A
+end
+
 function fillband!(A::AdjOrTrans, v, k1, k2)
     fillband!(parent(A), wrapperop(A)(v), -k2, -k1)
     return A

src/dense.jl

@@ -235,6 +235,8 @@ function fillband!(A::AbstractMatrix{T}, x, l, u) where T
     return A
 end
 
+fillstored!(A::AbstractMatrix, v) = fill!(A, v)
+
 diagind(m::Integer, n::Integer, k::Integer=0) = diagind(IndexLinear(), m, n, k)
 diagind(::IndexLinear, m::Integer, n::Integer, k::Integer=0) =
     k <= 0 ? range(1-k, step=m+1, length=min(m+k, n)) : range(k*m+1, step=m+1, length=min(m, n-k))

test/adjtrans.jl

@@ -810,6 +810,17 @@ end
     end
 end
 
+@testset "fillstored!" begin
+    A = rand(ComplexF64, 4, 4)
+    U = UpperTriangular(A)
+    @testset for (op, f) in ((Adjoint, adjoint), (Transpose, transpose))
+        @test LinearAlgebra.fillstored!(op(A), 1) == op(fill(1, size(A)))
+        @test LinearAlgebra.fillstored!(op(A), 2im) == op(fill(f(2im), size(A)))
+        @test LinearAlgebra.fillstored!(op(U), 1) == op(triu(fill(1, size(U))))
+        @test LinearAlgebra.fillstored!(op(U), 2im) == op(triu(fill(f(2im), size(U))))
+    end
+end
+        
 @testset "lmul!/rmul! by numbers" begin
     @testset "$(eltype(A))" for A in (rand(4, 4), rand(ComplexF64,4,4),
                 fill([1 2; 3 4], 4, 4),