fix: handle empty idxs in interpolation

Author: AayushSabharwal

src/solutions/ode_solutions.jl

@@ -245,6 +245,9 @@ end
 function (sol::AbstractODESolution)(t::Number, ::Type{deriv},
         idxs::AbstractVector{<:Integer},
         continuity) where {deriv}
+    if isempty(idxs)
+        return eltype(eltype(sol.u))[]
+    end
     if eltype(sol.u) <: Number
         idxs = only(idxs)
     end
@@ -259,6 +262,9 @@ end
 function (sol::AbstractODESolution)(t::AbstractVector{<:Number}, ::Type{deriv},
         idxs::AbstractVector{<:Integer},
         continuity) where {deriv}
+    if isempty(idxs)
+        return map(_ -> eltype(eltype(sol.u))[], t)
+    end
     if eltype(sol.u) <: Number
         idxs = only(idxs)
     end
@@ -295,6 +301,9 @@ function (sol::AbstractODESolution)(t::Number, ::Type{deriv}, idxs::AbstractVect
        any(isequal(NotSymbolic()), symbolic_type.(idxs))
         error("Incorrect specification of `idxs`")
     end
+    if isempty(idxs)
+        return eltype(eltype(sol.u))[]
+    end
     error_if_observed_derivative(sol, idxs, deriv)
     ps = parameter_values(sol)
     if is_parameter_timeseries(sol) == Timeseries() && is_discrete_expression(sol, idxs)
@@ -335,6 +344,9 @@ end
 
 function (sol::AbstractODESolution)(t::AbstractVector{<:Number}, ::Type{deriv},
         idxs::AbstractVector, continuity) where {deriv}
+    if isempty(idxs)
+        return map(_ -> eltype(eltype(sol.u))[], t)
+    end
     error_if_observed_derivative(sol, idxs, deriv)
     p = hasproperty(sol.prob, :p) ? sol.prob.p : nothing
     getter = getu(sol, idxs)

test/solution_interface.jl

@@ -37,3 +37,17 @@ end
     @test_throws ErrorException sol(-0.5)
     @test_throws ErrorException sol([0, -0.5, 0])
 end
+
+@testset "interpolate with empty idxs" begin
+    f = (u, p, t) -> u
+    sol1 = SciMLBase.build_solution(
+        ODEProblem(f, 1.0, (0.0, 1.0)), :NoAlgorithm, 0.0:0.1:1.0, exp.(0.0:0.1:1.0))
+    sol2 = SciMLBase.build_solution(ODEProblem(f, [1.0, 2.0], (0.0, 1.0)), :NoAlgorithm,
+        0.0:0.1:1.0, vcat.(exp.(0.0:0.1:1.0), 2exp.(0.0:0.1:1.0)))
+    for sol in [sol1, sol2]
+        @test sol(0.15; idxs = []) == Float64[]
+        @test sol(0.15; idxs = Int[]) == Float64[]
+        @test sol([0.15, 0.25]; idxs = []) == [Float64[], Float64[]]
+        @test sol([0.15, 0.25]; idxs = Int[]) == [Float64[], Float64[]]
+    end
+end