Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.83%. Comparing base (959d985) to head (9072335).
⚠️ Report is 270 commits behind head on master.

@@            Coverage Diff             @@
##           master    #1158      +/-   ##
==========================================
- Coverage   91.84%   91.83%   -0.01%     
==========================================
  Files          34       34              
  Lines       15357    15358       +1     
==========================================
  Hits        14104    14104              
- Misses       1253     1254       +1     

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I'm unsure if diag(D) and diag(D,0) returning different types is a good idea. The specific conversion issue may be solved by simply copying the diagonal vector in the constructor:

Diagonal(A::AbstractMatrix) = Diagonal(_diagcopy(A))
Diagonal{T}(A::AbstractMatrix) where T = Diagonal{T}(_diagcopy(A))
Diagonal{T,V}(A::AbstractMatrix) where {T,V<:AbstractVector{T}} = Diagonal{T,V}(_diagcopy(A))

_diagcopy(D) = diag(D)
_diagcopy(D::Diagonal) = copy(D.diag)

with this,

julia> oftype(D2,D1)
5×5 Diagonal{Float64, StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}}:
 1.0   ⋅    ⋅    ⋅    ⋅ 
  ⋅   2.0   ⋅    ⋅    ⋅ 
  ⋅    ⋅   3.0   ⋅    ⋅ 
  ⋅    ⋅    ⋅   4.0   ⋅ 
  ⋅    ⋅    ⋅    ⋅   5.0

We may still want this, but let's be sure that it isn't for a quick fix.

Maybe zero-step ranges could be returned as substitutions of FillArrays.Fill. Then diag(::Diagonal{_,<:AbstractRange},k) becomes type-stable. For other lazy vectors, an interface similarzeros could be created.

I think JuliaLang/julia#51475 may help with that

I think JuliaLang/julia#51475 may help with that

sort of. We need zero ranges with different lengths here.

#1424 would offer an alternative to this.