Allow diagm(v) for a vector v

[dpsanders]

It would be useful to allow diagm(v) for a vector v, for the common case of wanting that vector on the main diagonal:

julia> using LinearAlgebra

julia> v = [3, 4]
2-element Array{Int64,1}:
 3
 4

julia> diagm(v)
ERROR: MethodError: no method matching diagm(::Array{Int64,1})
Closest candidates are:
  diagm(::Pair{#s623,#s622} where #s622<:(AbstractArray{T,1} where T) where #s623<:Integer...) at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.1/LinearAlgebra/src/dense.jl:291
Stacktrace:
 [1] top-level scope at none:0

julia> diagm(0=>v)
2×2 Array{Int64,2}:
 3  0
 0  4

[ararslan]

This method was explicitly deprecated in JuliaLang/julia#24047, so cc @fredrikekre

[dpsanders]

I guess the "correct" solution is Diagonal(v).

[StefanKarpinski]

Eh, I don't see why we need to be fussy about this. Let's just allow diagm(v) as a shorthand for the very common case diagm(0 => v).

[eulerkochy]

Are you implying to create a sort-of alias for diagm(v)?

[StefanKarpinski]

No, just to define the missing method.

[ChanchalKumarMaji]

Hello, I was planning to contribute towards GSoC 2019. I want to contribute to this issue. Please help me how to start.

[StefanKarpinski]

Thanks for the interest, @ChanchalKumarMaji. There is already a PR implementing this: JuliaLang/julia#31125.

[ChanchalKumarMaji]

Ok, thanks for the response, I will look into another issue.

[karthikrangasai]

@StefanKarpinski , I just submitted a PR for this issue JuliaLang/julia#31210

[StefanKarpinski]

That's great, however, as I said, there's already a pull request implementing this feature that has gone through several iterations.