Scalar indexing on `mul!(::SubArray, ::Number, ::UniformScaling)`

[jonas-schulze]

The Minimal Working Example (MWE) for this bug:

using CUDA
using LinearAlgebra
using LinearAlgebra: checksquare

# Adapted from uniformscaling.jl:287 as of Julia 1.10.9
# TODO: Discuss this change upstream:
function LinearAlgebra.mul!(out::SubArray{T, 2, <: CuMatrix{T}}, a::Number, B::UniformScaling, α::Number, β::Number) where {T}
    checksquare(out)
    if iszero(β)  # zero contribution of the out matrix
        fill!(out, zero(T))
    elseif !isone(β)
        rmul!(out, β)
    end
    s = convert(T, a*B.λ*α)
    if !iszero(s)
        ids = diagind(out)
        out[ids] .+= s # allocates :-(
    end
    return out
end

n = 100
X = CUDA.randn(n, n)
Xv = @view X[1:n, 1:n]

@assert Xv isa SubArray
mul!(Xv, 42, I) # boom

All the rest is identical to #3041.
First appeared in #3041 (comment).