[Bug] Importing solution fails when OCP has exactly one variable #857

using OptimalControl
using NLPModelsIpopt
using JLD2

filename="sol"

## Fine
tf = 1
ocp = @def begin
    t ∈ [0, tf], time
    x ∈ R, state
    u ∈ R, control
    x(0) == 0
    x(tf) == 1
    ẋ(t) == u(t)
    ∫(0.5u(t)^2) → min
end

s = solve(ocp, grid_size=8)
export_ocp_solution(s; filename)
si = import_ocp_solution(ocp; filename)

## Workaround with dummy parameter p_
ocp = @def begin
    p = (tf, p_) ∈ R², variable
    t ∈ [0, tf], time
    x ∈ R, state
    u ∈ R, control
    p_ == 0
    1 <= tf <= 2
    x(0) == 0
    x(tf) == 1
    ẋ(t) == u(t)
    ∫(0.5u(t)^2) → min
end

s = solve(ocp, grid_size=8)
export_ocp_solution(s; filename)
si = import_ocp_solution(ocp; filename)

## Bug
ocp = @def begin
    tf ∈ R, variable
    t ∈ [0, tf], time
    x ∈ R, state
    u ∈ R, control
    1 <= tf <= 2
    x(0) == 0
    x(tf) == 1
    ẋ(t) == u(t)
    ∫(0.5u(t)^2) → min
end

s = solve(ocp, grid_size=8)
export_ocp_solution(s; filename)
si = import_ocp_solution(ocp; filename)