Sum identity and matrix of variables

[blegat]

julia> model = Model()
@vA JuMP Model
Feasibility problem with:
Variables: 0
ariModel mode: AUTOMATIC
CachingOptimizer state: NO_OPTIMIZER
Solver name: No optimizer attached.

julia> @variable(model, Q[1:2, 1:2], PSD)
2×2 LinearAlgebra.Symmetric{VariableRef,Array{VariableRef,2}}:
 Q[1,1]  Q[1,2]
 Q[1,2]  Q[2,2]

julia> using LinearAlgebra
Q 
julia> Q + I
ERROR: MethodError: Union{}(::VariableRef) is ambiguous. Candidates:
  (::Type{SA})(x...) where SA<:StaticArrays.StaticArray in StaticArrays at /home/blegat/.julia/packages/StaticArrays/VyRz3/src/convert.jl:4
  (::Type{T})(itr) where T<:Tuple in Base at tuple.jl:246
Possible fix, define
  Union{}(::Any)
Stacktrace:
 [1] convert(::Type{Union{}}, ::VariableRef) at ./sysimg.jl:71
 [2] setindex!(::Array{Union{},2}, ::VariableRef, ::Int64) at ./array.jl:767
 [3] copyto! at ./abstractarray.jl:753 [inlined]
 [4] copyto! at ./abstractarray.jl:745 [inlined]
 [5] AbstractArray{Union{},2}(::Array{VariableRef,2}) at ./array.jl:483
 [6] convert at ./abstractarray.jl:16 [inlined]
 [7] AbstractArray{Union{},2}(::Symmetric{VariableRef,Array{VariableRef,2}}) at /build/julia/src/julia-1.1.1/usr/share/julia/stdlib/v1.1/LinearAlgebra/src/symmetric.jl:256
 [8] convert(::Type{AbstractArray{Union{},2}}, ::Symmetric{VariableRef,Array{VariableRef,2}}) at ./abstractarray.jl:16
 [9] copy_oftype(::Symmetric{VariableRef,Array{VariableRef,2}}, ::Type{Union{}}) at /build/julia/src/julia-1.1.1/usr/share/julia/stdlib/v1.1/LinearAlgebra/src/LinearAlgebra.jl:341
 [10] +(::Symmetric{VariableRef,Array{VariableRef,2}}, ::UniformScaling{Bool}) at /build/julia/src/julia-1.1.1/usr/share/julia/stdlib/v1.1/LinearAlgebra/src/uniformscaling.jl:117
 [11] top-level scope at none:0

(v1.1) pkg> st JuMP
    Status `~/.julia/environments/v1.1/Project.toml`
  [4076af6c] JuMP v0.19.0
  [b8f27783] MathOptInterface v0.8.4

[blegat]

Apparently, we need to define the sum between an affine expression and a UniformScaling, so this should work too

julia> @variable(model, x)
x

julia> x + I
ERROR: MethodError: no method matching +(::VariableRef, ::UniformScaling{Bool})
Closest candidates are:
  +(::Any, ::Any, ::Any, ::Any...) at operators.jl:502
  +(::BitArray{2}, ::UniformScaling) at /build/julia/src/julia-1.1.1/usr/share/julia/stdlib/v1.1/LinearAlgebra/src/uniformscaling.jl:90
  +(::UnitUpperTriangular, ::UniformScaling) at /build/julia/src/julia-1.1.1/usr/share/julia/stdlib/v1.1/LinearAlgebra/src/uniformscaling.jl:106
  ...
Stacktrace:
 [1] top-level scope at none:0