equality `==` of Solutions stack overflow

[anandijain]

# ==(::sol, ::sol) stackoverflow MWE
using DifferentialEquations

A = [2 -5; 1 -2]

function f(u, p, t)
  A*u
end

function f2!(du, u, p, t) 
  du .= A*u
end


function f3(u, p, t)
  A*u .* 3
end


u0 = [1, 1]
tspan = (0., 10.)


prob = ODEProblem(f, u0, tspan, [])
sol = solve(prob)

prob2 = ODEProblem(f2!, u0, tspan, [])
sol2 = solve(prob2)

prob3 = ODEProblem(f3, u0, tspan, [])
sol3 = solve(prob3)

sol == sol2 # stack overflow
sol == sol3 # false, no error

stacktrace:

julia> sol == sol2
ERROR: StackOverflowError:
Stacktrace:
 [1] ==(::OrdinaryDiffEq.ODECompositeSolution{Float64,2,Array{Array{Float64,1},1},Nothing,Nothing,Array{Float64,1},Array{Array{Array{Float64,1},1},1},ODEProblem{Array{Float64,1},Tuple{Float64,Float64},true,Array{Any,1},ODEFunction{true,typeof(f2!),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}},DiffEqBase.StandardODEProblem},CompositeAlgorithm{Tuple{Tsit5,Rosenbrock23{0,false,DefaultLinSolve,DataType}},OrdinaryDiffEq.AutoSwitchCache{Tsit5,Rosenbrock23{0,false,DefaultLinSolve,DataType},Rational{Int64},Int64}},OrdinaryDiffEq.CompositeInterpolationData{ODEFunction{true,typeof(f2!),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Array{Array{Float64,1},1},Array{Float64,1},Array{Array{Array{Float64,1},1},1},OrdinaryDiffEq.CompositeCache{Tuple{OrdinaryDiffEq.Tsit5Cache{Array{Float64,1},Array{Float64,1},Array{Float64,1},OrdinaryDiffEq.Tsit5ConstantCache{Float64,Float64}},OrdinaryDiffEq.Rosenbrock23Cache{Array{Float64,1},Array{Float64,1},Array{Float64,1},Array{Float64,2},Array{Float64,2},OrdinaryDiffEq.Rosenbrock23Tableau{Float64},DiffEqBase.TimeGradientWrapper{ODEFunction{true,typeof(f2!),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Array{Float64,1},Array{Any,1}},DiffEqBase.UJacobianWrapper{ODEFunction{true,typeof(f2!),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Float64,Array{Any,1}},DefaultLinSolve,FiniteDiff.JacobianCache{Array{Float64,1},Array{Float64,1},Array{Float64,1},UnitRange{Int64},Nothing,Val{:forward}(),Float64},FiniteDiff.GradientCache{Nothing,Array{Float64,1},Array{Float64,1},Float64,Val{:forward}(),Float64,Val{true}()}}},OrdinaryDiffEq.AutoSwitchCache{Tsit5,Rosenbrock23{0,false,DefaultLinSolve,DataType},Rational{Int64},Int64}}},DiffEqBase.DEStats}, ::OrdinaryDiffEq.ODECompositeSolution{Float64,2,Array{Array{Float64,1},1},Nothing,Nothing,Array{Float64,1},Array{Array{Array{Float64,1},1},1},ODEProblem{Array{Float64,1},Tuple{Float64,Float64},false,Array{Any,1},ODEFunction{false,typeof(f),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}},DiffEqBase.StandardODEProblem},CompositeAlgorithm{Tuple{Tsit5,Rosenbrock23{0,false,DefaultLinSolve,DataType}},OrdinaryDiffEq.AutoSwitchCache{Tsit5,Rosenbrock23{0,false,DefaultLinSolve,DataType},Rational{Int64},Int64}},OrdinaryDiffEq.CompositeInterpolationData{ODEFunction{false,typeof(f),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Array{Array{Float64,1},1},Array{Float64,1},Array{Array{Array{Float64,1},1},1},OrdinaryDiffEq.CompositeCache{Tuple{OrdinaryDiffEq.Tsit5ConstantCache{Float64,Float64},OrdinaryDiffEq.Rosenbrock23ConstantCache{Float64,DiffEqBase.TimeDerivativeWrapper{ODEFunction{false,typeof(f),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Array{Float64,1},Array{Any,1}},DiffEqBase.UDerivativeWrapper{ODEFunction{false,typeof(f),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Float64,Array{Any,1}},Array{Float64,2},LinearAlgebra.LU{Float64,Array{Float64,2}},DefaultLinSolve}},OrdinaryDiffEq.AutoSwitchCache{Tsit5,Rosenbrock23{0,false,DefaultLinSolve,DataType},Rational{Int64},Int64}}},DiffEqBase.DEStats}) at ./abstractarray.jl:1859
 [2] ==(::OrdinaryDiffEq.ODECompositeSolution{Float64,2,Array{Array{Float64,1},1},Nothing,Nothing,Array{Float64,1},Array{Array{Array{Float64,1},1},1},ODEProblem{Array{Float64,1},Tuple{Float64,Float64},true,Array{Any,1},ODEFunction{true,typeof(f2!),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}},DiffEqBase.StandardODEProblem},CompositeAlgorithm{Tuple{Tsit5,Rosenbrock23{0,false,DefaultLinSolve,DataType}},OrdinaryDiffEq.AutoSwitchCache{Tsit5,Rosenbrock23{0,false,DefaultLinSolve,DataType},Rational{Int64},Int64}},OrdinaryDiffEq.CompositeInterpolationData{ODEFunction{true,typeof(f2!),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Array{Array{Float64,1},1},Array{Float64,1},Array{Array{Array{Float64,1},1},1},OrdinaryDiffEq.CompositeCache{Tuple{OrdinaryDiffEq.Tsit5Cache{Array{Float64,1},Array{Float64,1},Array{Float64,1},OrdinaryDiffEq.Tsit5ConstantCache{Float64,Float64}},OrdinaryDiffEq.Rosenbrock23Cache{Array{Float64,1},Array{Float64,1},Array{Float64,1},Array{Float64,2},Array{Float64,2},OrdinaryDiffEq.Rosenbrock23Tableau{Float64},DiffEqBase.TimeGradientWrapper{ODEFunction{true,typeof(f2!),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Array{Float64,1},Array{Any,1}},DiffEqBase.UJacobianWrapper{ODEFunction{true,typeof(f2!),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Float64,Array{Any,1}},DefaultLinSolve,FiniteDiff.JacobianCache{Array{Float64,1},Array{Float64,1},Array{Float64,1},UnitRange{Int64},Nothing,Val{:forward}(),Float64},FiniteDiff.GradientCache{Nothing,Array{Float64,1},Array{Float64,1},Float64,Val{:forward}(),Float64,Val{true}()}}},OrdinaryDiffEq.AutoSwitchCache{Tsit5,Rosenbrock23{0,false,DefaultLinSolve,DataType},Rational{Int64},Int64}}},DiffEqBase.DEStats}, ::OrdinaryDiffEq.ODECompositeSolution{Float64,2,Array{Array{Float64,1},1},Nothing,Nothing,Array{Float64,1},Array{Array{Array{Float64,1},1},1},ODEProblem{Array{Float64,1},Tuple{Float64,Float64},false,Array{Any,1},ODEFunction{false,typeof(f),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}},DiffEqBase.StandardODEProblem},CompositeAlgorithm{Tuple{Tsit5,Rosenbrock23{0,false,DefaultLinSolve,DataType}},OrdinaryDiffEq.AutoSwitchCache{Tsit5,Rosenbrock23{0,false,DefaultLinSolve,DataType},Rational{Int64},Int64}},OrdinaryDiffEq.CompositeInterpolationData{ODEFunction{false,typeof(f),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Array{Array{Float64,1},1},Array{Float64,1},Array{Array{Array{Float64,1},1},1},OrdinaryDiffEq.CompositeCache{Tuple{OrdinaryDiffEq.Tsit5ConstantCache{Float64,Float64},OrdinaryDiffEq.Rosenbrock23ConstantCache{Float64,DiffEqBase.TimeDerivativeWrapper{ODEFunction{false,typeof(f),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Array{Float64,1},Array{Any,1}},DiffEqBase.UDerivativeWrapper{ODEFunction{false,typeof(f),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Float64,Array{Any,1}},Array{Float64,2},LinearAlgebra.LU{Float64,Array{Float64,2}},DefaultLinSolve}},OrdinaryDiffEq.AutoSwitchCache{Tsit5,Rosenbrock23{0,false,DefaultLinSolve,DataType},Rational{Int64},Int64}}},DiffEqBase.DEStats}) at ./abstractarray.jl:1865 (repeats 4058 times)
 [3] top-level scope at REPL[22]:1

[ChrisRackauckas]

We haven't defined a function here, so it's just doing the fallback behavior when they are the same type. We should probably add this overload.