one argument indexing not defined for SymTridiagonal, Bidiagonal, Symmetric and Hermitian

[jakebolewski]

julia> r = randn(10,10)
julia>  r = (r' * r) ./ 2
julia> t = SymTridiagonal(r)
julia> t[1]
ERROR: indexing not defined for Base.LinAlg.SymTridiagonal{Float64}
 in getindex at abstractarray.jl:434

[andreasnoack]

You are having a party tonight!

I'm wondering if we should define a fallback linear indexing method for AbstractArray. That would also fix #9991.

[jakebolewski]

Updated the title to reflect missing indexing functionality. Most methods that takeAbstractArrays as arguments rely on indexing behavior so this is an essential interface for AbstractArray subtypes. Defining a fallback would seem to be the way to go.

[andreasnoack]

I think two argument indexing is defined for all the types (if it wasn't they couldn't get printed), so maybe the title should say "One argument indexing...". That would also explain why a fall back would work.

[mbauman]

Fixed by #10525.