axpby! no longer works with Hermitian matrices

[KristofferC]

using LinearAlgebra
a = rand(5,5)
A = Hermitian(a' * a)
B = similar(a)
LinearAlgebra.BLAS.axpby!(1, A, 1.0, B)

1.7:

julia> LinearAlgebra.BLAS.axpby!(1, A, 1.0, B)
5×5 Matrix{Float64}:
 1.66822   1.02419   0.774382  1.26736   1.48034

1.8:

julia> LinearAlgebra.BLAS.axpby!(1, A, 1.0, B)
ERROR: MethodError: no method matching strides(::Hermitian{Float64, Matrix{Float64}})
Closest candidates are:
  strides(::SubArray) at subarray.jl:356
  strides(::Base.ReinterpretArray{T, N, S, A, true} where {T, N, S, A<:(AbstractArray{S})}) at reinterpretarray.jl:155
  strides(::Base.ReinterpretArray{T, N, S, A, false} where {T, N, S, A<:AbstractArray{S, N}}) at reinterpretarray.jl:165
  ...
Stacktrace:
 [1] vec_pointer_stride
   @ ~/julia/usr/share/julia/stdlib/v1.8/LinearAlgebra/src/blas.jl:173 [inlined]
 [2] vec_pointer_stride
   @ ~/julia/usr/share/julia/stdlib/v1.8/LinearAlgebra/src/blas.jl:172 [inlined]
 [3] axpby!(alpha::Int64, x::Hermitian{Float64, Matrix{Float64}}, beta::Float64, y::Matrix{Float64})
   @ LinearAlgebra.BLAS ~/julia/usr/share/julia/stdlib/v1.8/LinearAlgebra/src/blas.jl:590
 [4] top-level scope
   @ REPL[9]:1

Probably introduced in #42957. cc @N5N3

[N5N3]

On 1.7, LinearAlgebra.BLAS.axpby!(1, A, 1.0, B) use a general fallback thus not BLAS based.
As axpby!(1.0, 1.:10, 1.0, 1.:10) fallbacks to BLAS-based fallback on 1.7. I'm not sure previous behavior has no problem.

julia> VERSION
v"1.7.2"

julia> axpby!(1.0, 1.:10, 1.0, 1.:10)
ERROR: MethodError: no method matching strides(::StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64})

I'd expect BLAS.f(....) always based on BLAS. Thus I suggest to distinguish BLAS.axpy! and LinearAlgebra.axpy!.
But I guess that change is not acceptable for 1.8.

[KristofferC]

Not sure, I just saw a PkgEval failure from it. But I agree that it makes sense to separate the BLAS and LinearAlgebra part in the long run. For now, is there an easy way to restore the functionality?

[N5N3]

PR opened.